diff --git a/roaring/roaring.go b/roaring/roaring.go
index e74739311..0a91c4493 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -890,9 +890,17 @@ func (c *container) bitmapCountRange(start, end uint32) int {
 	var n uint64
 	i, j := start/64, end/64
 
+	// Special case when start and end fall in the same word.
+	if i == j {
+		offi, offj := start%64, 64-end%64
+		n += popcount((c.bitmap[i] >> offi) << (offj + offi))
+		return int(n)
+	}
+
 	// Count partial starting word.
 	if off := start % 64; off != 0 {
-		n += popcount(c.bitmap[i] << off)
+		n += popcount(c.bitmap[i] >> off)
+		i++
 	}
 
 	// Count words in between.
@@ -902,9 +910,8 @@ func (c *container) bitmapCountRange(start, end uint32) int {
 
 	// Count partial ending word.
 	if int(j) < len(c.bitmap) {
-		if off := end % 64; off != 0 {
-			n += popcount(c.bitmap[j] >> off)
-		}
+		off := 64 - (end % 64)
+		n += popcount(c.bitmap[j] << off)
 	}
 
 	return int(n)
diff --git a/roaring/roaring_internal_test.go b/roaring/roaring_internal_test.go
new file mode 100644
index 000000000..5cff4a30f
--- /dev/null
+++ b/roaring/roaring_internal_test.go
@@ -0,0 +1,29 @@
+package roaring
+
+import (
+	"testing"
+)
+
+func TestBitmapCountRange(t *testing.T) {
+	c := container{}
+	tests := []struct {
+		start  uint32
+		end    uint32
+		bitmap []uint64
+		exp    int
+	}{
+		{start: 0, end: 1, bitmap: []uint64{1}, exp: 1},
+		{start: 2, end: 7, bitmap: []uint64{0xFFFFFFFFFFFFFF18}, exp: 2},
+		{start: 67, end: 68, bitmap: []uint64{0, 0x8}, exp: 1},
+		{start: 1, end: 68, bitmap: []uint64{0x3, 0x8, 0xF}, exp: 2},
+		{start: 1, end: 258, bitmap: []uint64{0xF, 0x8, 0xA, 0x4, 0xFFFFFFFFFFFFFFFF}, exp: 9},
+		{start: 66, end: 71, bitmap: []uint64{0xF, 0xFFFFFFFFFFFFFF18}, exp: 2},
+		{start: 63, end: 64, bitmap: []uint64{0x8000000000000000}, exp: 1},
+	}
+	for i, test := range tests {
+		c.bitmap = test.bitmap
+		if ret := c.bitmapCountRange(test.start, test.end); ret != test.exp {
+			t.Fatalf("test #%v count of %v from %v to %v should be %v but got %v", i, test.bitmap, test.start, test.end, test.exp, ret)
+		}
+	}
+}