diff --git a/edify/expr.c b/edify/expr.c
index 3600075..07a8ceb 100644
--- a/edify/expr.c
+++ b/edify/expr.c
@@ -495,7 +495,7 @@
 
 // Use printf-style arguments to compose an error message to put into
 // *state.  Returns NULL.
-Value* ErrorAbort(State* state, char* format, ...) {
+Value* ErrorAbort(State* state, const char* format, ...) {
     char* buffer = malloc(4096);
     va_list v;
     va_start(v, format);