diff --git a/fec/src/ir.c b/fec/src/ir.c
index 6b5f117..5ee0072 100644
--- a/fec/src/ir.c
+++ b/fec/src/ir.c
@@ -5,7 +5,7 @@
 void fe_ir_module_init(FeIrModule *m)
 {
     fe_arena_init(&m->arena, 16384);
-    m->unit_file = "";
+    m->file_count = 0;
     m->entry_main = 0;
     m->funcs = 0;
     m->last_func = 0;
@@ -13,6 +13,20 @@ void fe_ir_module_init(FeIrModule *m)
     m->last_global = 0;
 }
 
+/* The index of this path in the module's file table, adding it if it is new.
+   Traps carry the index rather than the string so the generator emits each
+   name once. */
+unsigned fe_ir_file(FeIrModule *m, const char *path)
+{
+    unsigned i;
+    if (!path) path = "";
+    for (i = 0; i < m->file_count; ++i)
+        if (!strcmp(m->files[i], path)) return i;
+    if (m->file_count >= FE_IR_FILE_MAX) return 0;
+    m->files[m->file_count] = path;
+    return m->file_count++;
+}
+
 void fe_ir_module_destroy(FeIrModule *m)
 {
     fe_arena_destroy(&m->arena);
@@ -301,13 +315,15 @@ void fe_ir_ret(FeIrBlock *b, unsigned value, int has_value)
     b->has_ret_value = has_value;
 }
 
-void fe_ir_trap(FeIrBlock *b, FeIrTrap reason, unsigned long line)
+void fe_ir_trap(FeIrBlock *b, FeIrTrap reason, unsigned long line,
+                unsigned file)
 {
     if (b->terminated) return;
     b->terminated = 1;
     b->term = FE_IR_TRAP;
     b->trap = reason;
     b->trap_line = line;
+    b->trap_file = file;
 }
 
 const char *fe_ir_type_name(FeIrType t)
@@ -432,8 +448,8 @@ void fe_ir_dump(const FeIrModule *m, FILE *out)
     const FeIrValue *v;
     const FeIrGlobal *g;
     unsigned i;
-    if (m->unit_file && m->unit_file[0])
-        fprintf(out, "; unit file %s\n", m->unit_file);
+    for (i = 0; i < m->file_count; ++i)
+        fprintf(out, "; file %u %s\n", i, m->files[i]);
     for (g = m->globals; g; g = g->next)
         fprintf(out, "global @%s : %s %lu\n", g->name,
                 fe_ir_type_name(g->type), g->size);
diff --git a/fec/src/ir.h b/fec/src/ir.h
index beff64f..8801844 100644
--- a/fec/src/ir.h
+++ b/fec/src/ir.h
@@ -4,6 +4,9 @@
 #include "arena.h"
 #include <stdio.h>
 
+/* How many source files one build can trap from. */
+#define FE_IR_FILE_MAX 64
+
 /* The intermediate representation. `IR.md` is the description; this is the
    shape it takes in memory.
 
@@ -88,6 +91,7 @@ typedef struct FeIrBlock {
     int has_ret_value;
     FeIrTrap trap;
     unsigned long trap_line;
+    unsigned trap_file;      /* index into the module's file table */
     /* Set once a terminator is chosen. Lowering asks before appending a
        jump, so a `return` inside a branch is not overwritten by the jump
        to the join block. */
@@ -142,7 +146,10 @@ typedef struct FeIrGlobal {
 
 typedef struct FeIrModule {
     FeArena arena;
-    const char *unit_file;   /* the one file-name string a unit's traps share */
+    /* Every unit in the build lands in one module, so a trap has to say which
+       file it came from rather than share one name with the whole program. */
+    const char *files[FE_IR_FILE_MAX];
+    unsigned file_count;
     /* The entry unit's `main`, if it has one. The runtime's start stub
        calls a fixed name, so the generator emits a jump to this one. */
     const char *entry_main;
@@ -152,6 +159,7 @@ typedef struct FeIrModule {
     FeIrGlobal *last_global;
 } FeIrModule;
 
+unsigned fe_ir_file(FeIrModule *m, const char *path);
 void fe_ir_module_init(FeIrModule *m);
 void fe_ir_module_destroy(FeIrModule *m);
 
@@ -198,7 +206,8 @@ void     fe_ir_copy(FeIrModule *m, FeIrBlock *b, FeIrPlace dst, FeIrPlace src,
 void fe_ir_jmp(FeIrBlock *b, unsigned target);
 void fe_ir_br(FeIrBlock *b, unsigned cond, unsigned t, unsigned f);
 void fe_ir_ret(FeIrBlock *b, unsigned value, int has_value);
-void fe_ir_trap(FeIrBlock *b, FeIrTrap reason, unsigned long line);
+void fe_ir_trap(FeIrBlock *b, FeIrTrap reason, unsigned long line,
+                unsigned file);
 
 void fe_ir_dump(const FeIrModule *m, FILE *out);
 const char *fe_ir_type_name(FeIrType t);
diff --git a/fec/src/lower.c b/fec/src/lower.c
index 41ff46e..dbfc439 100644
--- a/fec/src/lower.c
+++ b/fec/src/lower.c
@@ -199,6 +199,13 @@ FeIrBlock *new_block(Lower *L)
     return fe_ir_block(L->m, L->fn);
 }
 
+/* Which file a trap raised right now came from. The whole build lowers into
+   one module, so the unit being lowered is the only thing that knows. */
+unsigned trap_file(Lower *L)
+{
+    return fe_ir_file(L->m, L->c->unit ? L->c->unit->path : "");
+}
+
 /* A check that must hold. `ok` is a condition; when it is false the program
    stops where it is. `--no-checks` removes the comparison and the branch, not
    just the message, which is the whole point of the flag. */
@@ -208,7 +215,7 @@ void guard(Lower *L, unsigned ok, FeIrTrap reason, unsigned long line)
     FeIrBlock *cont = new_block(L);
     fe_ir_br(L->b, ok, cont->id, bad->id);
     L->b = bad;
-    fe_ir_trap(L->b, reason, line);
+    fe_ir_trap(L->b, reason, line, trap_file(L));
     L->b = cont;
 }
 
@@ -403,13 +410,13 @@ int lower_builtin(Lower *L, FeNode *n, Slot *out)
     const char *name = n->text;
     if (!name || name[0] != '@') return 0;
     if (!strcmp(name, "@trap")) {
-        fe_ir_trap(L->b, FE_TRAP_EXPLICIT, n->loc.line);
+        fe_ir_trap(L->b, FE_TRAP_EXPLICIT, n->loc.line, trap_file(L));
         L->b = new_block(L);
         *out = slot_void();
         return 1;
     }
     if (!strcmp(name, "@unreachable")) {
-        fe_ir_trap(L->b, FE_TRAP_UNREACHABLE, n->loc.line);
+        fe_ir_trap(L->b, FE_TRAP_UNREACHABLE, n->loc.line, trap_file(L));
         L->b = new_block(L);
         *out = slot_void();
         return 1;
diff --git a/fec/src/lowerpri.h b/fec/src/lowerpri.h
index 9ebd779..6b87c6b 100644
--- a/fec/src/lowerpri.h
+++ b/fec/src/lowerpri.h
@@ -121,6 +121,7 @@ unsigned declare_var(Lower *L, const char *cname, const FeType *t,
 int release_flag(Lower *L, unsigned local, unsigned *flag);
 LowerVar *find_var(Lower *L, const char *cname);
 FeIrBlock *new_block(Lower *L);
+unsigned trap_file(Lower *L);
 void guard(Lower *L, unsigned ok, FeIrTrap reason, unsigned long line);
 FeIrType tag_type(const FeType *t);
 int uses_niche(const FeType *t);
diff --git a/fec/src/lowerstm.c b/fec/src/lowerstm.c
index df09f18..cfc5f5d 100644
--- a/fec/src/lowerstm.c
+++ b/fec/src/lowerstm.c
@@ -631,7 +631,6 @@ int fe_lower_program(FeCheck *c, FeIrModule *out)
         c->ast = &unit->ast;
         c->unit = unit;
         c->types.unit_name = unit->name[0] ? unit->name : "unit";
-        if (!out->unit_file || !out->unit_file[0]) out->unit_file = unit->path;
         for (n = unit->ast.root ? unit->ast.root->children : 0; n; n = n->next)
             if (n->kind == FE_N_GLOBAL || n->kind == FE_N_CONST)
                 lower_global(&L, n);
diff --git a/fec/src/x86.c b/fec/src/x86.c
index 98e5824..69315b7 100644
--- a/fec/src/x86.c
+++ b/fec/src/x86.c
@@ -548,7 +548,8 @@ static void emit_func(const FeIrModule *m, const FeIrFunc *f, FILE *out)
             break;
         case FE_IR_TRAP:
             fprintf(out, "        push    %lu\n", b->trap_line);
-            fprintf(out, "        push    offset FE_UNIT_FILE\n");
+            fprintf(out, "        push    offset FE_FILE_%u\n",
+                    b->trap_file);
             fprintf(out, "        push    %u\n", (unsigned)b->trap);
             fprintf(out, "        call    fe_trap\n");
             fprintf(out, "        add     esp, 12\n");
@@ -587,6 +588,7 @@ void fe_x86_emit(const FeIrModule *m, FILE *out)
     const FeIrGlobal *g;
     int any_trap = 0;
     const FeIrBlock *b;
+    unsigned i;
 
     for (f = m->funcs; f && !any_trap; f = f->next)
         for (b = f->first; b; b = b->next)
@@ -623,9 +625,12 @@ void fe_x86_emit(const FeIrModule *m, FILE *out)
     if (any_trap) fputs("extern fe_trap : near\n", out);
 
     fputs("\n_DATA segment dword public 'DATA'\n", out);
-    if (any_trap) {
-        fputs("public FE_UNIT_FILE\nFE_UNIT_FILE label byte\n", out);
-        emit_string(m->unit_file, out);
+    /* One name per file a trap can come from. A build is many units in
+       one module, and a trap that names the wrong file is worse than
+       one that names none. */
+    for (i = 0; i < m->file_count; ++i) {
+        fprintf(out, "public FE_FILE_%u\nFE_FILE_%u label byte\n", i, i);
+        emit_string(m->files[i], out);
     }
     for (g = m->globals; g; g = g->next) {
         unsigned long i;
diff --git a/fec/tests/exec/lexer/parse.fe b/fec/tests/exec/lexer/parse.fe
index ffd5d2b..50f759d 100644
--- a/fec/tests/exec/lexer/parse.fe
+++ b/fec/tests/exec/lexer/parse.fe
@@ -23,10 +23,8 @@ pub struct Parser {
     pub errors: usize,
 
     pub fn on(src: []u8) -> !Self {
-        let nodes: list.List(ast.Node) =
-            try list.List(ast.Node).with_capacity(16);
         var p: Self = Self{
-            nodes: nodes,
+            nodes: try list.List(ast.Node).with_capacity(16),
             at: 0,
             line: 1,
             cur: tok.Token{ kind: tok.Kind.End, from: 0, len: 0, line: 1 },
diff --git a/fec/tests/exec/trapsrc/main.fe b/fec/tests/exec/trapsrc/main.fe
new file mode 100644
index 0000000..12f89d7
--- /dev/null
+++ b/fec/tests/exec/trapsrc/main.fe
@@ -0,0 +1,20 @@
+// EXIT:3
+// OUTPUT:index out of bounds
+// OUTPUT:pick.fe:6
+// NOCHECKS:0
+unit main;
+
+import pick;
+
+// A build is many units in one module. A trap has to name the file the check
+// was written in, not the file the program was started from.
+
+const S: str = "abc";
+
+fn main() -> i32 {
+    let c: u8 = pick.at(S, 9);
+    // Without the checks the read runs off the end and `c` is whatever
+    // was there, so nothing is decided by its value.
+    if c == 0 { return 0; }
+    return 0;
+}
diff --git a/fec/tests/exec/trapsrc/pick.fe b/fec/tests/exec/trapsrc/pick.fe
new file mode 100644
index 0000000..628d989
--- /dev/null
+++ b/fec/tests/exec/trapsrc/pick.fe
@@ -0,0 +1,7 @@
+unit pick;
+
+// The failing index is here, two units away from the program that runs it.
+
+pub fn at(s: []u8, i: usize) -> u8 {
+    return s[i];
+}