diff --git a/lib/rb/ext/compact_protocol.c b/lib/rb/ext/compact_protocol.c
index 7966d3e..33a1f9f 100644
--- a/lib/rb/ext/compact_protocol.c
+++ b/lib/rb/ext/compact_protocol.c
@@ -86,7 +86,7 @@
   } else {
     char str[50];
     sprintf(str, "don't know what type: %d", type);
-    rb_raise(rb_eStandardError, str);
+    rb_raise(rb_eStandardError, "%s", str);
     return 0;
   }
 }
@@ -348,7 +348,7 @@
   } else {
     char str[50];
     sprintf(str, "don't know what type: %d", ctype);
-    rb_raise(rb_eStandardError, str);
+    rb_raise(rb_eStandardError, "%s", str);
     return 0;
   }
 }