diff --git a/doc/fixes34.1 b/doc/fixes34.1
index bdff0262d..12e960f40 100644
--- a/doc/fixes34.1
+++ b/doc/fixes34.1
@@ -280,6 +280,7 @@ document "sound" option in Guidebook
 destroy traps that are buried by boulders dropped in water
 renamed debug commands: light sources -> lightsources,
 	monpoly_control -> monpolycontrol
+detect attempt to swap places with big pet through narrow opening
 
 
 Platform- and/or Interface-Specific Fixes
diff --git a/src/hack.c b/src/hack.c
index 739ef502c..6731ebe4a 100644
--- a/src/hack.c
+++ b/src/hack.c
@@ -1279,6 +1279,13 @@ domove()
 		    sobj_at(BOULDER, trap->tx, trap->ty)) {
 		/* can't swap places with pet pinned in a pit by a boulder */
 		u.ux = u.ux0,  u.uy = u.uy0;	/* didn't move after all */
+	    } else if (u.ux0 != x && u.uy0 != y &&
+		       bad_rock(mtmp->data, x, u.uy0) &&
+		       bad_rock(mtmp->data, u.ux0, y) &&
+		       (bigmonst(mtmp->data) || (curr_mon_load(mtmp) > 600))) {
+		/* can't swap places when pet won't fit thru the opening */
+		u.ux = u.ux0,  u.uy = u.uy0;	/* didn't move after all */
+		You("stop.  %s won't fit through.", upstart(y_monnam(mtmp)));
 	    } else {
 		char pnambuf[BUFSZ];