diff --git a/server/core/externcmd.c b/server/core/externcmd.c
index 8a4ae5451..874a41570 100644
--- a/server/core/externcmd.c
+++ b/server/core/externcmd.c
@@ -217,7 +217,9 @@ bool externcmd_substitute_arg(EXTERNCMD* cmd, const char* match, const char* rep
     {
         for (int i = 0; cmd->argv[i] && rval; i++)
         {
-            size_t size = strlen(cmd->argv[i]);
+            size_t size_orig = strlen(cmd->argv[i]);
+            size_t size_replace = strlen(replace);
+            size_t size = MXS_MAX(size_orig, size_replace);
             char* dest = MXS_MALLOC(size);
             if (dest)
             {