The primary issue here is that the current code is returning
the initialized (empty) reply to asKeys requests.

However, I have other questions:

Even asKeys requests arrive with a resWindow back reference.
Since these don't expect a response, I suppose you would process
this even if resWindow == None. But currently, even asExpr requests
are processed when there is no resWindow.
Why/when would this need to be done?

When would you receive a request and name != serverName?
For that matter, when would serverName == NULL?
These requests are not being processed, but if there is a resWindow,
then a reply is being sent - for asKeys or asExpr. And that reply
would indicate e_invexprmsg, which really isn't the case.

--- a/src/if_xcmdsrv.c	Thu Jul 05 00:12:17 2012 -0400
+++ b/src/if_xcmdsrv.c	Thu Jul 05 02:20:43 2012 -0400
@@ -1197,9 +1197,8 @@
 	if ((*p == 'c' || *p == 'k') && (p[1] == 0))
 	{
 	    Window	resWindow;
-	    char_u	*name, *script, *serial, *end, *res;
+	    char_u	*name, *script, *serial, *end;
 	    Bool	asKeys = *p == 'k';
-	    garray_T	reply;
 	    char_u	*enc;

 	    /*
@@ -1255,50 +1254,47 @@
 	    if (script == NULL || name == NULL)
 		continue;

-	    /*
-	     * Initialize the result property, so that we're ready at any
-	     * time if we need to return an error.
-	     */
-	    if (resWindow != None)
-	    {
-		ga_init2(&reply, 1, 100);
+            if (serverName != NULL && STRICMP(name, serverName) == 0)
+            {
+                script = serverConvert(enc, script, &tofree);
+                if (asKeys)
+                    server_to_input_buf(script);
+                else if (resWindow != None)
+                {
+                    char_u      *res = NULL;
+                    garray_T    reply;
+
+                    /* Initialize the result property. */
+                    ga_init2(&reply, 1, 100);
 #ifdef FEAT_MBYTE
-		ga_grow(&reply, 50 + STRLEN(p_enc));
-		sprintf(reply.ga_data, "%cr%c-E %s%c-s %s%c-r ",
-						   0, 0, p_enc, 0, serial, 0);
-		reply.ga_len = 14 + STRLEN(p_enc) + STRLEN(serial);
+                    ga_grow(&reply, 50 + STRLEN(p_enc));
+                    sprintf(reply.ga_data, "%cr%c-E %s%c-s %s%c-r ",
+                                                    0, 0, p_enc, 0, serial, 0);
+                    reply.ga_len = 14 + STRLEN(p_enc) + STRLEN(serial);
 #else
-		ga_grow(&reply, 50);
-		sprintf(reply.ga_data, "%cr%c-s %s%c-r ", 0, 0, serial, 0);
-		reply.ga_len = 10 + STRLEN(serial);
+                    ga_grow(&reply, 50);
+                    sprintf(reply.ga_data, "%cr%c-s %s%c-r ", 0, 0, serial, 0);
+                    reply.ga_len = 10 + STRLEN(serial);
 #endif
-	    }
-	    res = NULL;
-	    if (serverName != NULL && STRICMP(name, serverName) == 0)
-	    {
-		script = serverConvert(enc, script, &tofree);
-		if (asKeys)
-		    server_to_input_buf(script);
-		else
-		    res = eval_client_expr_to_string(script);
-		vim_free(tofree);
-	    }
-	    if (resWindow != None)
-	    {
-		if (res != NULL)
-		    ga_concat(&reply, res);
-		else if (asKeys == 0)
-		{
-		    ga_concat(&reply, (char_u *)_(e_invexprmsg));
-		    ga_append(&reply, 0);
-		    ga_concat(&reply, (char_u *)"-c 1");
-		}
-		ga_append(&reply, NUL);
-		(void)AppendPropCarefully(dpy, resWindow, commProperty,
-					   reply.ga_data, reply.ga_len);
-		ga_clear(&reply);
-	    }
-	    vim_free(res);
+
+                    /* Evaluate the expression and return the result. */
+                    if ((res = eval_client_expr_to_string(script)) != NULL)
+                        ga_concat(&reply, res);
+                    else
+                    {
+                        ga_concat(&reply, (char_u *)_(e_invexprmsg));
+                        ga_append(&reply, 0);
+                        ga_concat(&reply, (char_u *)"-c 1");
+                    }
+                    ga_append(&reply, NUL);
+                    (void)AppendPropCarefully(dpy, resWindow, commProperty,
+                                                reply.ga_data, reply.ga_len);
+
+                    ga_clear(&reply);
+                    vim_free(res);
+                }
+                vim_free(tofree);
+            }
 	}
 	else if (*p == 'r' && p[1] == 0)
 	{