diff --git a/all.h b/all.h
index 8519434..99609d9 100644
--- a/all.h
+++ b/all.h
@@ -677,7 +677,7 @@ fn Codepoint codepointFromBytesBefore(ptr bytes, u32 offset);
 fn Codepoint codepointFromRawInt(u32 c);
 fn void codepointFillBuf(Codepoint cp, ptr buf);
 fn String stringFromRawCodepoint(Arena* a, u32 c);
-fn bool stringInsertCodepointAtByte(String* s, Codepoint c, u32 byte_offset);
+fn bool stringInsertCodepointAtByte(String* s, Codepoint c, u32 byte_offset, bool keep_trailing_zero);
 fn u8 stringDeleteCodepointAtByte(String* s, u32 byte_offset);
 fn bool stringDeleteCodepointsBetweenByteOffsetsInclusive(String* s, u32 start, u32 end);
 
diff --git a/string.c b/string.c
index 14120ce..57659c2 100644
--- a/string.c
+++ b/string.c
@@ -244,8 +244,8 @@ fn String stringFromRawCodepoint(Arena* a, u32 c) {
   return result;
 }
 
-fn bool stringInsertCodepointAtByte(String* s, Codepoint c, u32 byte_offset) {
-  u32 remaining_space = s->capacity - s->length;
+fn bool stringInsertCodepointAtByte(String* s, Codepoint c, u32 byte_offset, bool keep_trailing_zero) {
+  i32 remaining_space = (s->capacity - (keep_trailing_zero ? 1 : 0)) - s->length;
   if (remaining_space < c.size) return false;
 
   char codepoint_bytes[4];