diff --git a/client/client_test.go b/client/client_test.go
index 4bdc9c1c9..bfb457624 100644
--- a/client/client_test.go
+++ b/client/client_test.go
@@ -43,6 +43,7 @@ func TestClientIntegration(t *testing.T) {
 		testWhiteoutParentDir,
 		testDuplicateWhiteouts,
 		testSchema1Image,
+		testMountWithNoSource,
 	})
 }
 
@@ -705,6 +706,37 @@ func testSchema1Image(t *testing.T, sb integration.Sandbox) {
 	checkAllReleasable(t, c, sb, true)
 }
 
+// #319
+func testMountWithNoSource(t *testing.T, sb integration.Sandbox) {
+	t.Parallel()
+	c, err := New(sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	busybox := llb.Image("docker.io/library/busybox:latest")
+	st := llb.Scratch()
+
+	var nilState llb.State
+
+	// This should never actually be run, but we want to succeed
+	// if it was, because we expect an error below, or a daemon
+	// panic if the issue has regressed.
+	run := busybox.Run(
+		llb.Args([]string{"/bin/true"}),
+		llb.AddMount("/nil", nilState, llb.SourcePath("/"), llb.Readonly))
+
+	st = run.AddMount("/mnt", st)
+
+	def, err := st.Marshal()
+	require.NoError(t, err)
+
+	err = c.Solve(context.TODO(), def, SolveOpt{}, nil)
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "has no input")
+
+	checkAllReleasable(t, c, sb, true)
+}
+
 func requiresLinux(t *testing.T) {
 	if runtime.GOOS != "linux" {
 		t.Skipf("unsupported GOOS: %s", runtime.GOOS)
diff --git a/executor/oci/spec_unix.go b/executor/oci/spec_unix.go
index 028e88892..ba7aa79d9 100644
--- a/executor/oci/spec_unix.go
+++ b/executor/oci/spec_unix.go
@@ -50,6 +50,9 @@ func GenerateSpec(ctx context.Context, meta executor.Meta, mounts []executor.Mou
 	sm := &submounts{}
 
 	for _, m := range mounts {
+		if m.Src == nil {
+			return nil, nil, errors.Errorf("mount %s has no source", m.Dest)
+		}
 		mounts, err := m.Src.Mount(ctx, m.Readonly)
 		if err != nil {
 			sm.cleanup()
diff --git a/solver/llbop/exec.go b/solver/llbop/exec.go
index 2b3fc2908..2e38924e0 100644
--- a/solver/llbop/exec.go
+++ b/solver/llbop/exec.go
@@ -95,6 +95,11 @@ func (e *execOp) Run(ctx context.Context, inputs []solver.Ref) ([]solver.Ref, er
 				mountable = active
 			}
 		}
+
+		if mountable == nil {
+			return nil, errors.Errorf("mount %s has no input", m.Dest)
+		}
+
 		if m.Dest == pb.RootMount {
 			root = mountable
 		} else {