diff --git a/common/coords.go b/common/coords.go
index 719073c..bd0d613 100644
--- a/common/coords.go
+++ b/common/coords.go
@@ -14,8 +14,8 @@ const (
 	numBitsPerComponent = 12
 	modulo              = 1 << numBitsPerComponent
 	maxPositive         = modulo / 2
-	minValue            = -1 << numBitsPerComponent
-	maxValue            = 1<<numBitsPerComponent - 1
+	minValue            = -1 << (numBitsPerComponent - 1)
+	maxValue            = 1<<(numBitsPerComponent-1) - 1
 )
 
 type (
@@ -272,13 +272,13 @@ func BigMin(minz, maxz, zcode int64) int64 {
 	pos := msb
 	for m := int64(1) << msb; m != 0; m >>= 1 {
 		var v uint8
-		if zcode&m != 0 {
+		if zcode&m == m {
 			v = _100_
 		}
-		if minz&m != 0 {
+		if minz&m == m {
 			v |= _010_
 		}
-		if maxz&m != 0 {
+		if maxz&m == m {
 			v |= _001_
 		}
 		switch v {
diff --git a/common/coords_test.go b/common/coords_test.go
index b6f9b81..519ac4a 100644
--- a/common/coords_test.go
+++ b/common/coords_test.go
@@ -181,7 +181,7 @@ func outsiders(zmin, zmax int64, fn func(int64)) {
 }
 
 func TestBigMin(t *testing.T) {
-	const tries = 10
+	const tries = 20
 	for i := 0; i < tries; i++ {
 		x1 := rand.Intn(4000) - 2000
 		y1 := rand.Intn(4000) - 2000