diff --git a/source/blender/blenlib/intern/string_cursor_utf8.c b/source/blender/blenlib/intern/string_cursor_utf8.c
index 83c9d159ca7..e768a042628 100644
--- a/source/blender/blenlib/intern/string_cursor_utf8.c
+++ b/source/blender/blenlib/intern/string_cursor_utf8.c
@@ -111,6 +111,8 @@ static eStrCursorDelimType cursor_delim_type_utf8(const char *ch_utf8,
 bool BLI_str_cursor_step_next_utf8(const char *str, const int str_maxlen, int *pos)
 {
   /* NOTE: Keep in sync with #BLI_str_cursor_step_next_utf32. */
+  BLI_assert(str_maxlen >= 0);
+  BLI_assert(*pos >= 0);
 
   if (*pos >= str_maxlen) {
     return false;
@@ -133,6 +135,8 @@ bool BLI_str_cursor_step_next_utf8(const char *str, const int str_maxlen, int *p
 bool BLI_str_cursor_step_prev_utf8(const char *str, const int str_maxlen, int *pos)
 {
   /* NOTE: Keep in sync with #BLI_str_cursor_step_prev_utf32. */
+  BLI_assert(str_maxlen >= 0);
+  BLI_assert(*pos >= 0);
 
   if ((*pos > 0) && (*pos <= str_maxlen)) {
     const char *str_pos = str + *pos;
@@ -230,6 +234,7 @@ bool BLI_str_cursor_step_next_utf32(const char32_t *str, const int str_maxlen, i
 {
   /* NOTE: Keep in sync with #BLI_str_cursor_step_next_utf8. */
   BLI_assert(str_maxlen >= 0);
+  BLI_assert(*pos >= 0);
 
   if (*pos >= str_maxlen) {
     return false;
@@ -245,6 +250,7 @@ bool BLI_str_cursor_step_prev_utf32(const char32_t *str, const int str_maxlen, i
 {
   /* NOTE: Keep in sync with #BLI_str_cursor_step_prev_utf8. */
   BLI_assert(str_maxlen >= 0);
+  BLI_assert(*pos >= 0);
   UNUSED_VARS_NDEBUG(str_maxlen);
 
   if (*pos <= 0) {