diff -r b3c4d168221a config.def.h
--- a/config.def.h	Sun Jul 06 20:27:17 2008 +0200
+++ b/config.def.h	Sun Jul 06 20:29:18 2008 +0200
@@ -27,7 +27,7 @@
 };
 
 /* layout(s) */
-static float mfact      = 0.55;
+#define MFACT 0.55
 static Bool resizehints = True; /* False means don't respect size hints in tiled resizals */
 
 static Layout layouts[] = {
diff -r b3c4d168221a dwm.c
--- a/dwm.c	Sun Jul 06 20:27:17 2008 +0200
+++ b/dwm.c	Sun Jul 06 20:29:18 2008 +0200
@@ -208,6 +208,7 @@
 static char stext[256];
 static int screen, sx, sy, sw, sh;
 static int by, bh, blw, wx, wy, ww, wh;
+static float mfact;
 static uint seltags = 0, sellt = 0;
 static int (*xerrorxlib)(Display *, XErrorEvent *);
 static uint numlockmask = 0;
@@ -1304,12 +1305,16 @@
 setmfact(const Arg *arg) {
 	float f;
 
-	if(!arg || !lt[sellt]->arrange)
+	if (!lt[sellt]->arrange)
 		return;
-	f = arg->f < 1.0 ? arg->f + mfact : arg->f - 1.0;
-	if(f < 0.1 || f > 0.9)
-		return;
-	mfact = f;
+	if(arg->f) {
+		f = arg->f < 1.0 ? arg->f + mfact : arg->f - 1.0;
+		if(f < 0.1 || f > 0.9)
+			return;
+		mfact = f;
+	}
+	else
+		mfact = MFACT;
 	arrange();
 }
 
@@ -1330,6 +1335,7 @@
 	bh = dc.h = dc.font.height + 2;
 	lt[0] = &layouts[0];
 	lt[1] = &layouts[1 % LENGTH(layouts)];
+	mfact = MFACT;
 	updategeom();
 
 	/* init atoms */