diff --git a/src/sync/map.go b/src/sync/map.go
index a61e2eb..9ad2535 100644
--- a/src/sync/map.go
+++ b/src/sync/map.go
@@ -274,6 +274,7 @@
 		e, ok = read.m[key]
 		if !ok && read.amended {
 			e, ok = m.dirty[key]
+			delete(m.dirty, key)
 			// Regardless of whether the entry was present, record a miss: this key
 			// will take the slow path until the dirty map is promoted to the read
 			// map.
diff --git a/src/sync/map_test.go b/src/sync/map_test.go
index 4ae989a..7f163ca 100644
--- a/src/sync/map_test.go
+++ b/src/sync/map_test.go
@@ -9,6 +9,7 @@
 	"reflect"
 	"runtime"
 	"sync"
+	"sync/atomic"
 	"testing"
 	"testing/quick"
 )
@@ -171,3 +172,26 @@
 		}
 	}
 }
+
+func TestIssue40999(t *testing.T) {
+	var m sync.Map
+
+	// Since the miss-counting in missLocked (via Delete)
+	// compares the miss count with len(m.dirty),
+	// add an initial entry to bias len(m.dirty) above the miss count.
+	m.Store(nil, struct{}{})
+
+	var finalized uint32
+
+	// Set finalizers that count for collected keys. A non-zero count
+	// indicates that keys have not been leaked.
+	for atomic.LoadUint32(&finalized) == 0 {
+		p := new(int)
+		runtime.SetFinalizer(p, func(*int) {
+			atomic.AddUint32(&finalized, 1)
+		})
+		m.Store(p, struct{}{})
+		m.Delete(p)
+		runtime.GC()
+	}
+}