diff --git a/roaring/roaring.go b/roaring/roaring.go
index e0810a6c2..1a4d8d486 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -904,9 +904,17 @@ 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)
+		i++
 	}
 
 	// Count words in between.
@@ -916,7 +924,7 @@ func (c *container) bitmapCountRange(start, end uint32) int {
 
 	// Count partial ending word.
 	if int(j) < len(c.bitmap) {
-		if off := end % 64; off != 0 {
+		if off := 64 - (end % 64); off != 0 {
 			n += popcount(c.bitmap[j] >> off)
 		}
 	}
diff --git a/roaring/roaring_internal_test.go b/roaring/roaring_internal_test.go
new file mode 100644
index 000000000..17a35c5e1
--- /dev/null
+++ b/roaring/roaring_internal_test.go
@@ -0,0 +1,31 @@
+package roaring
+
+import (
+	"testing"
+)
+
+func TestBitmapCountRange(t *testing.T) {
+	c := container{bitmap: []uint64{1}}
+	cnt := c.bitmapCountRange(63, 65)
+	if cnt != 1 {
+		t.Fatalf("count of %v from 63 to 65 should be 1, but got %v", c.bitmap, cnt)
+	}
+
+	c = container{bitmap: []uint64{0, 0x8000000000000000}}
+	cnt = c.bitmapCountRange(65, 66)
+	if cnt != 0 {
+		t.Fatalf("count of %v from 65 to 66 should be 0, but got %v", c.bitmap, cnt)
+	}
+
+	c = container{bitmap: []uint64{0, 0xF000000000000000}}
+	cnt = c.bitmapCountRange(65, 66)
+	if cnt != 1 {
+		t.Fatalf("count of %v from 65 to 66 should be 1, but got %v", c.bitmap, cnt)
+	}
+
+	c = container{bitmap: []uint64{0x1, 0xFF00000000000000}}
+	cnt = c.bitmapCountRange(62, 66)
+	if cnt != 3 {
+		t.Fatalf("count of %v from 62 to 66 should be 3, but got %v", c.bitmap, cnt)
+	}
+}