diff --git a/weed/s3api/policy_conversion.go b/weed/s3api/policy_conversion.go
index a93b87cfa..722e379cf 100644
--- a/weed/s3api/policy_conversion.go
+++ b/weed/s3api/policy_conversion.go
@@ -78,7 +78,9 @@ func convertPrincipal(principal interface{}) *policy_engine.StringOrStringSlice
 		// Convert []interface{} to []string
 		strs = make([]string, 0, len(p))
 		for _, v := range p {
-			strs = append(strs, convertToString(v))
+			if v != nil {
+				strs = append(strs, convertToString(v))
+			}
 		}
 	case map[string]interface{}:
 		// Handle AWS-style principal with service/user keys
@@ -91,7 +93,9 @@ func convertPrincipal(principal interface{}) *policy_engine.StringOrStringSlice
 				strs = append(strs, val...)
 			case []interface{}:
 				for _, item := range val {
-					strs = append(strs, convertToString(item))
+					if item != nil {
+						strs = append(strs, convertToString(item))
+					}
 				}
 			}
 		}
@@ -135,7 +139,9 @@ func convertConditionValue(value interface{}) policy_engine.StringOrStringSlice
 	case []interface{}:
 		strs := make([]string, 0, len(v))
 		for _, item := range v {
-			strs = append(strs, convertToString(item))
+			if item != nil {
+				strs = append(strs, convertToString(item))
+			}
 		}
 		return policy_engine.NewStringOrStringSlice(strs...)
 	default:
diff --git a/weed/s3api/policy_conversion_test.go b/weed/s3api/policy_conversion_test.go
index 2d00fbdfd..6ea5d48c2 100644
--- a/weed/s3api/policy_conversion_test.go
+++ b/weed/s3api/policy_conversion_test.go
@@ -195,3 +195,96 @@ func TestConvertPrincipalUnknownType(t *testing.T) {
 	}
 }
 
+func TestConvertPrincipalWithNilValues(t *testing.T) {
+	// Test that nil values in arrays are skipped for security
+	principalArray := []interface{}{
+		"arn:aws:iam::123456789012:user/Alice",
+		nil, // Should be skipped
+		"arn:aws:iam::123456789012:user/Bob",
+		nil, // Should be skipped
+	}
+
+	result := convertPrincipal(principalArray)
+	
+	if result == nil {
+		t.Fatal("Expected non-nil result")
+	}
+
+	strs := result.Strings()
+	// Should only have 2 values (nil values skipped)
+	if len(strs) != 2 {
+		t.Errorf("Expected 2 values (nil values skipped), got %d", len(strs))
+	}
+
+	expectedValues := []string{
+		"arn:aws:iam::123456789012:user/Alice",
+		"arn:aws:iam::123456789012:user/Bob",
+	}
+
+	for i, expected := range expectedValues {
+		if strs[i] != expected {
+			t.Errorf("Value[%d]: expected '%s', got '%s'", i, expected, strs[i])
+		}
+	}
+}
+
+func TestConvertConditionValueWithNilValues(t *testing.T) {
+	// Test that nil values in condition arrays are skipped
+	mixedValues := []interface{}{
+		"string",
+		nil, // Should be skipped
+		123,
+		nil, // Should be skipped
+		true,
+	}
+
+	result := convertConditionValue(mixedValues)
+	strs := result.Strings()
+
+	// Should only have 3 values (nil values skipped)
+	expectedValues := []string{"string", "123", "true"}
+	if len(strs) != len(expectedValues) {
+		t.Errorf("Expected %d values (nil values skipped), got %d", len(expectedValues), len(strs))
+	}
+
+	for i, expected := range expectedValues {
+		if strs[i] != expected {
+			t.Errorf("Value[%d]: expected '%s', got '%s'", i, expected, strs[i])
+		}
+	}
+}
+
+func TestConvertPrincipalMapWithNilValues(t *testing.T) {
+	// Test AWS-style principal map with nil values
+	principalMap := map[string]interface{}{
+		"AWS": []interface{}{
+			"arn:aws:iam::123456789012:user/Alice",
+			nil, // Should be skipped
+			"arn:aws:iam::123456789012:user/Bob",
+		},
+	}
+
+	result := convertPrincipal(principalMap)
+	
+	if result == nil {
+		t.Fatal("Expected non-nil result")
+	}
+
+	strs := result.Strings()
+	// Should only have 2 values (nil value skipped)
+	if len(strs) != 2 {
+		t.Errorf("Expected 2 values (nil value skipped), got %d", len(strs))
+	}
+
+	expectedValues := []string{
+		"arn:aws:iam::123456789012:user/Alice",
+		"arn:aws:iam::123456789012:user/Bob",
+	}
+
+	for i, expected := range expectedValues {
+		if strs[i] != expected {
+			t.Errorf("Value[%d]: expected '%s', got '%s'", i, expected, strs[i])
+		}
+	}
+}
+