diff --git a/string.c b/string.c
index 9b6961c392..96115e0ce0 100644
--- a/string.c
+++ b/string.c
@@ -5897,7 +5897,7 @@ typedef struct mapping_buffer {
     size_t capa;
     size_t used;
     struct mapping_buffer *next;
-    OnigUChar space[0];
+    OnigUChar space[1];
 } mapping_buffer;
 
 static VALUE
@@ -5923,7 +5923,7 @@ rb_str_casemap(VALUE source, OnigCaseFoldType *flags, rb_encoding *enc)
 	if (CASEMAP_DEBUG) {
 	    fprintf(stderr, "Buffer allocation, capa is %"PRIuSIZE"\n", capa); /* for tuning */
 	}
-	current_buffer->next = (mapping_buffer*)ALLOC_N(char, sizeof(mapping_buffer)+capa);
+	current_buffer->next = (mapping_buffer*)ALLOC_N(char, offsetof(mapping_buffer, space)+capa);
 	current_buffer = current_buffer->next;
 	current_buffer->next = NULL;
 	current_buffer->capa = capa;