diff --git a/server/filestore.go b/server/filestore.go
index 20f6657917..67c0932485 100644
--- a/server/filestore.go
+++ b/server/filestore.go
@@ -581,6 +581,9 @@ func (fs *fileStore) UpdateConfig(cfg *StreamConfig) error {
 	if cfg.Storage != FileStorage {
 		return fmt.Errorf("fileStore requires file storage type in config")
 	}
+	if cfg.MaxMsgsPer < -1 {
+		cfg.MaxMsgsPer = -1
+	}
 
 	fs.mu.Lock()
 	new_cfg := FileStreamInfo{Created: fs.cfg.Created, StreamConfig: *cfg}
@@ -611,7 +614,7 @@ func (fs *fileStore) UpdateConfig(cfg *StreamConfig) error {
 		fs.ageChk = nil
 	}
 
-	if fs.cfg.MaxMsgsPer > 0 && fs.cfg.MaxMsgsPer < old_cfg.MaxMsgsPer {
+	if fs.cfg.MaxMsgsPer > 0 && (old_cfg.MaxMsgsPer == 0 || fs.cfg.MaxMsgsPer < old_cfg.MaxMsgsPer) {
 		fs.enforceMsgPerSubjectLimit(true)
 	}
 	fs.mu.Unlock()
diff --git a/server/memstore.go b/server/memstore.go
index b183d117da..b83f62535f 100644
--- a/server/memstore.go
+++ b/server/memstore.go
@@ -85,10 +85,13 @@ func (ms *memStore) UpdateConfig(cfg *StreamConfig) error {
 		ms.ageChk = nil
 	}
 	// Make sure to update MaxMsgsPer
+	if cfg.MaxMsgsPer < -1 {
+		cfg.MaxMsgsPer = -1
+	}
 	maxp := ms.maxp
 	ms.maxp = cfg.MaxMsgsPer
-	// If the value is smaller we need to enforce that.
-	if ms.maxp != 0 && ms.maxp < maxp {
+	// If the value is smaller, or was unset before, we need to enforce that.
+	if ms.maxp > 0 && (maxp == 0 || ms.maxp < maxp) {
 		lm := uint64(ms.maxp)
 		ms.fss.Iter(func(subj []byte, ss *SimpleState) bool {
 			if ss.Msgs > lm {
diff --git a/server/store_test.go b/server/store_test.go
index d2c3481a1a..19d4c0251f 100644
--- a/server/store_test.go
+++ b/server/store_test.go
@@ -235,3 +235,47 @@ func TestStoreSubjectStateConsistency(t *testing.T) {
 		},
 	)
 }
+
+func TestStoreMaxMsgsPerUpdateBug(t *testing.T) {
+	config := func() StreamConfig {
+		return StreamConfig{Name: "TEST", Subjects: []string{"foo"}, MaxMsgsPer: 0}
+	}
+	testAllStoreAllPermutations(
+		t, false, config(),
+		func(t *testing.T, fs StreamStore) {
+			for i := 0; i < 5; i++ {
+				_, _, err := fs.StoreMsg("foo", nil, nil)
+				require_NoError(t, err)
+			}
+
+			ss := fs.State()
+			require_Equal(t, ss.Msgs, 5)
+			require_Equal(t, ss.FirstSeq, 1)
+			require_Equal(t, ss.LastSeq, 5)
+
+			// Update max messages per-subject from 0 (infinite) to 1.
+			// Since the per-subject limit was not specified before, messages should be removed upon config update.
+			cfg := config()
+			if _, ok := fs.(*fileStore); ok {
+				cfg.Storage = FileStorage
+			} else {
+				cfg.Storage = MemoryStorage
+			}
+			cfg.MaxMsgsPer = 1
+			err := fs.UpdateConfig(&cfg)
+			require_NoError(t, err)
+
+			// Only one message should remain.
+			ss = fs.State()
+			require_Equal(t, ss.Msgs, 1)
+			require_Equal(t, ss.FirstSeq, 5)
+			require_Equal(t, ss.LastSeq, 5)
+
+			// Update max messages per-subject from 0 (infinite) to an invalid value (< -1).
+			cfg.MaxMsgsPer = -2
+			err = fs.UpdateConfig(&cfg)
+			require_NoError(t, err)
+			require_Equal(t, cfg.MaxMsgsPer, -1)
+		},
+	)
+}