diff --git a/src/textprop.c b/src/textprop.c
index f219b92..a976414 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -695,7 +695,7 @@
 	    semsg(_("E969: Property type %s already defined"), name);
 	    return;
 	}
-	prop = alloc_clear(sizeof(proptype_T) + STRLEN(name));
+	prop = alloc_clear(offsetof(proptype_T, pt_name) + STRLEN(name) + 1);
 	if (prop == NULL)
 	    return;
 	STRCPY(prop->pt_name, name);