diff --git a/shared/nm-glib-aux/nm-macros-internal.h b/shared/nm-glib-aux/nm-macros-internal.h
index d5a8513e54..6b7285e470 100644
--- a/shared/nm-glib-aux/nm-macros-internal.h
+++ b/shared/nm-glib-aux/nm-macros-internal.h
@@ -929,19 +929,24 @@ nm_streq0 (const char *s1, const char *s2)
 
 #define NM_STR_HAS_PREFIX(str, prefix) \
 	({ \
-		const char *const _str = (str); \
+		const char *const _str_has_prefix = (str); \
 		\
-		_str && (strncmp ((str), ""prefix"", NM_STRLEN (prefix)) == 0); \
+		nm_assert (strlen (prefix) == NM_STRLEN (prefix)); \
+		\
+		   _str_has_prefix \
+		&& (strncmp (_str_has_prefix, ""prefix"", NM_STRLEN (prefix)) == 0); \
 	})
 
 #define NM_STR_HAS_SUFFIX(str, suffix) \
 	({ \
-		const char *_str; \
+		const char *const _str_has_suffix = (str); \
 		gsize _l; \
 		\
-		(   (_str = (str)) \
-		 && ((_l = strlen (_str)) >= NM_STRLEN (suffix)) \
-		 && (memcmp (&_str[_l - NM_STRLEN (suffix)], \
+		nm_assert (strlen (suffix) == NM_STRLEN (suffix)); \
+		\
+		(   _str_has_suffix \
+		 && ((_l = strlen (_str_has_suffix)) >= NM_STRLEN (suffix)) \
+		 && (memcmp (&_str_has_suffix[_l - NM_STRLEN (suffix)], \
 		             ""suffix"", \
 		             NM_STRLEN (suffix)) == 0)); \
 	})