--- modules/math/pdf_stat.tcl
+++ modules/math/pdf_stat.tcl
@@ -80,11 +80,11 @@
 	return -code error -errorcode ARG \
 		-errorinfo "Wrong order or zero range" \
 		"Wrong order or zero range"
     }
 
-    set prob [expr {1.0/($pmax-$min)}]
+    set prob [expr {1.0/($pmax-$pmin)}]
 
     if { $x < $pmin || $x > $pmax } { return 0.0 }
 
     return $prob
 }

