diff --git a/FileCheck.xs b/FileCheck.xs
index d9d38d3..e23c242 100644
--- a/FileCheck.xs
+++ b/FileCheck.xs
@@ -66,6 +66,7 @@ OverloadFTOps  *gl_overload_ft = 0;
 *
 *  1 check is true  -> OP returns Yes
 *  0 check is false -> OP returns No
+* -2 check is null  -> OP returns undef (CHECK_IS_NULL)
 * -1 fallback to the original OP
 */
 int _overload_ft_ops() {
@@ -93,7 +94,13 @@ int _overload_ft_ops() {
   if (count != 1)
     croak("No return value from Overload::FileCheck::_check for OP #%d\n", optype);
 
-  check_status = POPi;
+  {
+    SV *result_sv = POPs;
+    if (!SvOK(result_sv))
+      check_status = -2;  /* undef => CHECK_IS_NULL */
+    else
+      check_status = SvIV(result_sv);
+  }
 
   OFC_DEBUG("_overload_ft_ops: result=%d optype=%d\n", check_status, optype);
 
@@ -258,8 +265,9 @@ PP(pp_overload_ft_yes_no) {
   {
     FT_SETUP_dSP_IF_NEEDED;
 
-    if ( check_status == 1 ) FT_RETURNYES;
-    if ( check_status == 0 ) FT_RETURNUNDEF;
+    if ( check_status == 1 )  FT_RETURNYES;
+    if ( check_status == 0 )  FT_RETURNUNDEF;
+    if ( check_status == -2 ) FT_RETURNUNDEF; /* CHECK_IS_NULL */
   }
 
   /* fallback */
@@ -281,6 +289,11 @@ PP(pp_overload_ft_int) {
   if ( check_status == -1 )
     return CALL_REAL_OP();
 
+  if ( check_status == -2 ) { /* CHECK_IS_NULL */
+    FT_SETUP_dSP_IF_NEEDED;
+    FT_RETURNUNDEF;
+  }
+
   /* Save errno — sv_setiv() and FT_RETURN_TARG can trigger allocations
    * or other Perl internals that clobber errno. */
   saved_errno = errno;
@@ -307,6 +320,12 @@ PP(pp_overload_ft_nv) {
 
   status = _overload_ft_ops_sv();
 
+  if ( !SvOK(status) ) { /* CHECK_IS_NULL — undef */
+    SvREFCNT_dec(status);
+    FT_SETUP_dSP_IF_NEEDED;
+    FT_RETURNUNDEF;
+  }
+
   /* Save errno — sv_setnv()/sv_setiv() and FT_RETURN_TARG can trigger
    * allocations or other Perl internals that clobber errno. */
   saved_errno = errno;
diff --git a/lib/Overload/FileCheck.pm b/lib/Overload/FileCheck.pm
index 6238a51..219dcc3 100644
--- a/lib/Overload/FileCheck.pm
+++ b/lib/Overload/FileCheck.pm
@@ -55,7 +55,7 @@ my @STAT_T_IX = qw{
   ST_BLOCKS
 };
 
-my @CHECK_STATUS = qw{CHECK_IS_FALSE CHECK_IS_TRUE FALLBACK_TO_REAL_OP};
+my @CHECK_STATUS = qw{CHECK_IS_FALSE CHECK_IS_TRUE CHECK_IS_NULL FALLBACK_TO_REAL_OP};
 
 my @STAT_HELPERS = qw{ stat_as_directory stat_as_file stat_as_symlink
   stat_as_socket stat_as_chr stat_as_block};
@@ -425,7 +425,7 @@ sub unmock_stat {
 
 sub unmock_all_file_checks {
 
-    my @mocks = map { $REVERSE_MAP{$_} } keys %$_current_mocks;
+    my @mocks = sort map { $REVERSE_MAP{$_} } keys %$_current_mocks;
     return unless scalar @mocks;
 
     return unmock_file_check(@mocks);
@@ -454,7 +454,16 @@ sub _check {
     # causing resource leaks (e.g. sockets staying open). See GH #179.
     $_last_call_for = ref($file) ? undef : $file;
 
-    if ( defined $out && $OP_CAN_RETURN_INT{$optype} ) {
+    if ( !defined $out ) {
+        # CHECK_IS_NULL: callback returned undef — propagate as undef
+        # so the OP returns undef to the caller (file not found / unknown)
+        if ( !int($!) ) {
+            $! = $DEFAULT_ERRNO{ $REVERSE_MAP{$optype} || 'default' } || $DEFAULT_ERRNO{'default'};
+        }
+        return CHECK_IS_NULL;
+    }
+
+    if ( $OP_CAN_RETURN_INT{$optype} ) {
         return $out;
     }
 
diff --git a/t/check-null.t b/t/check-null.t
new file mode 100644
index 0000000..b692655
--- /dev/null
+++ b/t/check-null.t
@@ -0,0 +1,206 @@
+#!/usr/bin/perl -w
+
+# Tests for CHECK_IS_NULL handling.
+# When a mock callback returns undef (CHECK_IS_NULL), the file-test OP
+# should return undef to the caller, distinct from CHECK_IS_FALSE (0).
+
+use strict;
+use warnings;
+
+use Test2::Bundle::Extended;
+use Test2::Tools::Explain;
+use Test2::Plugin::NoWarnings;
+
+use Overload::FileCheck q(:all);
+
+# ---------------------------------------------------------------
+# Boolean ops (-e, -f, -d, …) — CHECK_IS_NULL should yield undef
+# ---------------------------------------------------------------
+
+{
+    note "boolean op: -e with CHECK_IS_NULL";
+
+    my $mock_return;
+
+    mock_file_check(
+        '-e' => sub { return $mock_return },
+    );
+
+    $mock_return = CHECK_IS_TRUE;
+    is( -e "/null-test", 1, "-e CHECK_IS_TRUE returns 1" );
+
+    $mock_return = CHECK_IS_FALSE;
+    ok( !defined( -e "/null-test" ), "-e CHECK_IS_FALSE returns undef (existing behaviour)" );
+
+    $mock_return = CHECK_IS_NULL;    # undef
+    ok( !defined( -e "/null-test" ), "-e CHECK_IS_NULL returns undef" );
+
+    # Ensure it is really undef, not 0 or ''
+    $mock_return = CHECK_IS_NULL;
+    my $result = -e "/null-test";
+    is( $result, undef, "-e CHECK_IS_NULL is literally undef" );
+
+    unmock_all_file_checks();
+}
+
+{
+    note "boolean op: -f with CHECK_IS_NULL";
+
+    mock_file_check(
+        '-f' => sub { return CHECK_IS_NULL },
+    );
+
+    my $result = -f "/null-test-f";
+    is( $result, undef, "-f CHECK_IS_NULL returns undef" );
+
+    unmock_all_file_checks();
+}
+
+{
+    note "boolean op: -d with CHECK_IS_NULL";
+
+    mock_file_check(
+        '-d' => sub { return CHECK_IS_NULL },
+    );
+
+    my $result = -d "/null-test-d";
+    is( $result, undef, "-d CHECK_IS_NULL returns undef" );
+
+    unmock_all_file_checks();
+}
+
+# ---------------------------------------------------------------
+# Integer op (-s) — CHECK_IS_NULL should yield undef, not 0
+# ---------------------------------------------------------------
+
+{
+    note "integer op: -s with CHECK_IS_NULL";
+
+    my $mock_return;
+
+    mock_file_check(
+        '-s' => sub { return $mock_return },
+    );
+
+    $mock_return = 42;
+    {
+        my $result = -s "/null-test-s";
+        is( $result, 42, "-s returns the mocked size" );
+    }
+
+    $mock_return = 0;
+    {
+        my $result = -s "/null-test-s";
+        is( $result, 0, "-s returns 0 when size is 0" );
+    }
+
+    $mock_return = CHECK_IS_NULL;    # undef
+    my $result = -s "/null-test-s";
+    is( $result, undef, "-s CHECK_IS_NULL returns undef (not 0)" );
+
+    unmock_all_file_checks();
+}
+
+# ---------------------------------------------------------------
+# NV ops (-M, -A, -C) — CHECK_IS_NULL should yield undef
+# ---------------------------------------------------------------
+
+{
+    note "NV op: -M with CHECK_IS_NULL";
+
+    my $mock_return;
+
+    mock_file_check(
+        '-M' => sub { return $mock_return },
+    );
+
+    $mock_return = 1.5;
+    ok( defined( -M "/null-test-M" ), "-M returns a defined value for 1.5" );
+
+    $mock_return = CHECK_IS_NULL;    # undef
+    my $result = -M "/null-test-M";
+    is( $result, undef, "-M CHECK_IS_NULL returns undef" );
+
+    unmock_all_file_checks();
+}
+
+{
+    note "NV op: -A with CHECK_IS_NULL";
+
+    mock_file_check(
+        '-A' => sub { return CHECK_IS_NULL },
+    );
+
+    my $result = -A "/null-test-A";
+    is( $result, undef, "-A CHECK_IS_NULL returns undef" );
+
+    unmock_all_file_checks();
+}
+
+{
+    note "NV op: -C with CHECK_IS_NULL";
+
+    mock_file_check(
+        '-C' => sub { return CHECK_IS_NULL },
+    );
+
+    my $result = -C "/null-test-C";
+    is( $result, undef, "-C CHECK_IS_NULL returns undef" );
+
+    unmock_all_file_checks();
+}
+
+# ---------------------------------------------------------------
+# errno is set when CHECK_IS_NULL is returned
+# ---------------------------------------------------------------
+
+{
+    note "errno handling with CHECK_IS_NULL";
+    local $! = 0;
+
+    mock_file_check(
+        '-e' => sub { return CHECK_IS_NULL },
+    );
+
+    my $result = -e "/null-errno-test";
+    ok( !defined $result, "result is undef" );
+    ok( int($!) > 0, "errno is set when CHECK_IS_NULL is returned" );
+
+    unmock_all_file_checks();
+}
+
+# ---------------------------------------------------------------
+# FALLBACK_TO_REAL_OP still works alongside CHECK_IS_NULL
+# ---------------------------------------------------------------
+
+{
+    note "FALLBACK_TO_REAL_OP alongside CHECK_IS_NULL";
+
+    mock_file_check(
+        '-e' => sub {
+            my $f = shift;
+            return CHECK_IS_NULL if $f eq '/check-null-missing';
+            return FALLBACK_TO_REAL_OP;
+        },
+    );
+
+    my $null_result = -e '/check-null-missing';
+    is( $null_result, undef, "CHECK_IS_NULL path returns undef" );
+
+    my $real_result = -e $0;
+    is( $real_result, 1, "FALLBACK_TO_REAL_OP path works for existing file" );
+
+    unmock_all_file_checks();
+}
+
+# ---------------------------------------------------------------
+# CHECK_IS_NULL constant value
+# ---------------------------------------------------------------
+
+{
+    note "CHECK_IS_NULL constant";
+
+    ok( !defined CHECK_IS_NULL, "CHECK_IS_NULL is undef" );
+}
+
+done_testing;