diff --git a/string.c b/string.c
index ecd6b97f88..b656f7ab0d 100644
--- a/string.c
+++ b/string.c
@@ -11763,6 +11763,9 @@ sym_inspect(VALUE sym)
         RB_GC_GUARD(orig_str);
     }
     dest[0] = ':';
+
+    RUBY_ASSERT(BUILTIN_TYPE(str) == T_STRING);
+
     return str;
 }