diff --git a/fmt/string.h b/fmt/string.h
index 2f487f4d..cb87b15e 100644
--- a/fmt/string.h
+++ b/fmt/string.h
@@ -8,7 +8,7 @@
  */
 
 #ifdef FMT_INCLUDE
-# error Add the fmt's parent directory and not fmt itself to includes.
+# error "Add the fmt's parent directory and not fmt itself to includes."
 #endif
 
 #ifndef FMT_STRING_H_