diff --git a/filter/config.Y b/filter/config.Y
index 726e13af..3a176df2 100644
--- a/filter/config.Y
+++ b/filter/config.Y
@@ -184,7 +184,7 @@ pair:
  * Complex types, their bison value is struct f_val
  */
 prefix_s:
-   IPA '/' NUM { $$.type = T_PREFIX; $$.val.px.ip = $1; $$.val.px.len = $3; cf_error( "%I/%d is not really prefix\n", $$.val.px.ip, $$.val.px.len ); }
+   IPA '/' NUM { $$.type = T_PREFIX; $$.val.px.ip = $1; $$.val.px.len = $3; if (ipa_nonzero(ipa_and($$.val.px.ip, ipa_not(ipa_mkmask($$.val.px.len))))) cf_error( "%I/%d is not really prefix\n", $$.val.px.ip, $$.val.px.len ); }
  ;
 
 prefix:
diff --git a/filter/test.conf b/filter/test.conf
index 7a1c07b0..799fc60c 100644
--- a/filter/test.conf
+++ b/filter/test.conf
@@ -38,11 +38,11 @@ ip p;
 	if 1234 = i then printn "."; else { print "*** FAIL: if 1 else"; }
 	if 1 <= 1 then printn "."; else { print "*** FAIL: test 3"; }
 	if 1234 < 1234 then { print "*** FAIL: test 4"; quitbird; } else print "ok";
-	print "  data types; must be true: " 1.2.3.4 = 1.2.3.4 "," 1 ~ [1,2,3] "," 5 ~ [1..20] "," 2 ~ [ 1, 2, 3 ] "," 5 ~ [ 4 .. 7 ] "," 1.2.3.4 ~ [ 1.2.3.3..1.2.3.5 ] "," 1.2.3.4 ~ 1.2.3.4/8 "," 1.2.3.4/8 ~ 1.2.3.4/8 "," 1.2.3.4/8 ~ [ 1.2.3.4/8+ ] "," 1.2.3.4/16 ~ [ 1.2.3.4/8{ 15 , 16 } ] "," defined(1) "," defined(1.2.3.4);
-	print "  data types: must be false: " 1 ~ [ 2, 3, 4 ] "," 5 ~ [ 2, 3, 4, 7..11 ] "," 1.2.3.4 ~ [ 1.2.3.3, 1.2.3.5 ] "," (1,2) > (2,2) "," (1,1) > (1,1) "," 1.2.3.4/8 ~ [ 1.2.3.4/8- ] "," 1.2.3.4/17 ~ [ 1.2.3.4/8{ 15 , 16 } ];
+	print "  data types; must be true: " 1.2.3.4 = 1.2.3.4 "," 1 ~ [1,2,3] "," 5 ~ [1..20] "," 2 ~ [ 1, 2, 3 ] "," 5 ~ [ 4 .. 7 ] "," 1.2.3.4 ~ [ 1.2.3.3..1.2.3.5 ] "," 1.2.3.4 ~ 1.0.0.0/8 "," 1.0.0.0/8 ~ 1.0.0.0/8 "," 1.0.0.0/8 ~ [ 1.0.0.0/8+ ] "," 1.2.0.0/16 ~ [ 1.0.0.0/8{ 15 , 16 } ] "," defined(1) "," defined(1.2.3.4);
+	print "  data types: must be false: " 1 ~ [ 2, 3, 4 ] "," 5 ~ [ 2, 3, 4, 7..11 ] "," 1.2.3.4 ~ [ 1.2.3.3, 1.2.3.5 ] "," (1,2) > (2,2) "," (1,1) > (1,1) "," 1.0.0.0/8 ~ [ 1.0.0.0/8- ] "," 1.2.0.0/17 ~ [ 1.0.0.0/8{ 15 , 16 } ];
 
-	px = 1.2.3.4/18;
-	print "Testing prefixes: 1.2.3.4/18 = " px;
+	px = 1.2.0.0/18;
+	print "Testing prefixes: 1.0.0.0/18 = " px;
 	p = 127.1.2.3;
 	print "Testing mask : 127.0.0.0 = " p.mask(8);
 	print "Testing pairs: (1,2) = " (1,2);