diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 46681b3..7f380f3 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -34,6 +34,7 @@ jobs:
         terraform:
           - '1.8.*'
           - '1.9.*'
+          - '1.10.*'
     steps:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
       - uses: actions/setup-go@0c52d547c9bc32b1aa3301fd7a9cb496313a4491 # v5.0.0
@@ -80,6 +81,7 @@ jobs:
         terraform:
           - '1.8.*'
           - '1.9.*'
+          - '1.10.*'
     steps:
       - uses: actions/checkout@a5ac7e51b41094c92402da3b24376905380afc29 # v4.1.6
       - uses: actions/setup-go@cdcb36043654635271a94b9a6d1392de5bb323a7 # v5.0.1
diff --git a/internal/provider/not_null_function.go b/internal/provider/not_null_function.go
index 6a4b356..84240eb 100644
--- a/internal/provider/not_null_function.go
+++ b/internal/provider/not_null_function.go
@@ -30,7 +30,7 @@ func (r NotNullFunction) Definition(_ context.Context, _ function.DefinitionRequ
 		Parameters: []function.Parameter{
 			function.DynamicParameter{
 				AllowNullValue:     true,
-				AllowUnknownValues: true,
+				AllowUnknownValues: false,
 				Description:        "The argument to check",
 				Name:               "argument",
 			},
@@ -47,5 +47,15 @@ func (r NotNullFunction) Run(ctx context.Context, req function.RunRequest, resp
 		return
 	}
 
-	resp.Error = function.ConcatFuncErrors(resp.Result.Set(ctx, !argument.IsNull()))
+	if argument.IsNull() {
+		resp.Error = resp.Result.Set(ctx, false)
+		return
+	}
+
+	if !argument.IsUnderlyingValueNull() {
+		resp.Error = resp.Result.Set(ctx, true)
+		return
+	}
+
+	resp.Error = function.ConcatFuncErrors(resp.Result.Set(ctx, false))
 }
diff --git a/internal/provider/not_null_function_test.go b/internal/provider/not_null_function_test.go
index 698de0e..65488f5 100644
--- a/internal/provider/not_null_function_test.go
+++ b/internal/provider/not_null_function_test.go
@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/hashicorp/go-version"
+	"github.com/hashicorp/terraform-plugin-testing/config"
 	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
 	"github.com/hashicorp/terraform-plugin-testing/tfversion"
 )
@@ -220,6 +221,43 @@ output "test" {
 	})
 }
 
+func TestNotNullFunction_compoundValidation(t *testing.T) {
+	t.Parallel()
+	resource.UnitTest(t, resource.TestCase{
+		TerraformVersionChecks: []tfversion.TerraformVersionCheck{
+			tfversion.SkipBelow(version.Must(version.NewVersion("1.2.0"))),
+		},
+		ProtoV6ProviderFactories: testAccProtoV6ProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: `
+variable "example_value_a" {
+  default     = null
+  description = "Example input A for validation."
+  type        = string
+}
+variable "example_value_b" {
+  default     = null
+  description = "Example input B for validation."
+  type        = string
+  validation {
+    condition = anytrue([
+      provider::assert::not_null(var.example_value_a),
+      provider::assert::not_null(var.example_value_b)
+    ])
+    error_message = "At least one of example_value_a or example_value_b must be provided."
+  }
+}
+				`,
+				ConfigVariables: config.Variables{
+					"example_value_b": config.StringVariable("example-format-value"),
+				},
+				Check: resource.ComposeAggregateTestCheckFunc(),
+			},
+		},
+	})
+}
+
 func TestNotNullFunction_falseCases(t *testing.T) {
 	t.Parallel()
 	resource.UnitTest(t, resource.TestCase{
@@ -227,6 +265,12 @@ func TestNotNullFunction_falseCases(t *testing.T) {
 			tfversion.SkipBelow(version.Must(version.NewVersion(MinimalRequiredTerraformVersion))),
 		},
 		ProtoV6ProviderFactories: testAccProtoV6ProviderFactories,
+		ExternalProviders: map[string]resource.ExternalProvider{
+			"wireguard": {
+				Source:            "OJFord/wireguard",
+				VersionConstraint: "0.3.1",
+			},
+		},
 		Steps: []resource.TestStep{
 			{
 				Config: `
@@ -235,6 +279,23 @@ locals {
 }
 output "test" {
   value = provider::assert::not_null(local.object)
+}
+				`,
+				Check: resource.ComposeAggregateTestCheckFunc(
+					resource.TestCheckOutput("test", "false"),
+				),
+			},
+			{
+				Config: `
+resource "wireguard_asymmetric_key" "main" {}
+
+data "wireguard_config_document" "main" {
+  private_key = wireguard_asymmetric_key.main.private_key
+}
+
+output "test" {
+  // .addresses is always null in this configuration
+  value = provider::assert::not_null(data.wireguard_config_document.main.addresses)
 }
 				`,
 				Check: resource.ComposeAggregateTestCheckFunc(
diff --git a/internal/provider/null_function.go b/internal/provider/null_function.go
index 3b4622e..7be6c30 100644
--- a/internal/provider/null_function.go
+++ b/internal/provider/null_function.go
@@ -30,7 +30,7 @@ func (r IsNullFunction) Definition(_ context.Context, _ function.DefinitionReque
 		Parameters: []function.Parameter{
 			function.DynamicParameter{
 				AllowNullValue:     true,
-				AllowUnknownValues: true,
+				AllowUnknownValues: false,
 				Description:        "The argument to check",
 				Name:               "argument",
 			},
@@ -47,10 +47,13 @@ func (r IsNullFunction) Run(ctx context.Context, req function.RunRequest, resp *
 		return
 	}
 
-	if argument.UnderlyingValue() == nil {
-		if err := resp.Result.Set(ctx, true); err == nil {
-			return
-		}
+	if argument.IsNull() {
+		resp.Error = resp.Result.Set(ctx, true)
+		return
+	}
+
+	if argument.IsUnderlyingValueNull() {
+		resp.Error = resp.Result.Set(ctx, true)
 		return
 	}
 
diff --git a/internal/provider/null_function_test.go b/internal/provider/null_function_test.go
index 8704d12..bc35576 100644
--- a/internal/provider/null_function_test.go
+++ b/internal/provider/null_function_test.go
@@ -7,11 +7,12 @@ import (
 	"testing"
 
 	"github.com/hashicorp/go-version"
+	"github.com/hashicorp/terraform-plugin-testing/config"
 	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
 	"github.com/hashicorp/terraform-plugin-testing/tfversion"
 )
 
-func TestIsNullFunction(t *testing.T) {
+func TestNullFunction(t *testing.T) {
 	t.Parallel()
 	resource.UnitTest(t, resource.TestCase{
 		TerraformVersionChecks: []tfversion.TerraformVersionCheck{
@@ -36,7 +37,81 @@ output "test" {
 	})
 }
 
-func TestIsNullFunction_falseCases(t *testing.T) {
+func TestNullFunction_crossObjectValidation(t *testing.T) {
+	t.Parallel()
+	resource.UnitTest(t, resource.TestCase{
+		TerraformVersionChecks: []tfversion.TerraformVersionCheck{
+			tfversion.SkipBelow(version.Must(version.NewVersion(MinimalRequiredTerraformVersion))),
+		},
+		ExternalProviders: map[string]resource.ExternalProvider{
+			"wireguard": {
+				Source:            "OJFord/wireguard",
+				VersionConstraint: "0.3.1",
+			},
+		},
+		ProtoV6ProviderFactories: testAccProtoV6ProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: `
+resource "wireguard_asymmetric_key" "main" {}
+
+data "wireguard_config_document" "main" {
+  private_key = wireguard_asymmetric_key.main.private_key
+}
+
+output "test" {
+  // .addresses is always null in this configuration
+  value = provider::assert::null(data.wireguard_config_document.main.addresses)
+}
+				`,
+				Check: resource.ComposeAggregateTestCheckFunc(
+					resource.TestCheckOutput("test", "true"),
+				),
+			},
+		},
+	})
+}
+
+func TestNullFunction_compoundValidation(t *testing.T) {
+	t.Parallel()
+	resource.UnitTest(t, resource.TestCase{
+		TerraformVersionChecks: []tfversion.TerraformVersionCheck{
+			tfversion.SkipBelow(version.Must(version.NewVersion("1.2.0"))),
+		},
+		ProtoV6ProviderFactories: testAccProtoV6ProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: `
+variable "example_value_a" {
+  default     = null
+  description = "Example option A for testing null validation."
+  type        = string
+}
+
+variable "example_value_b" {
+  default     = null
+  description = "Example option B for testing null validation."
+  type        = string
+
+  validation {
+    condition = anytrue([
+      !provider::assert::null(var.example_value_a),
+      !provider::assert::null(var.example_value_b)
+    ])
+    error_message = "Exactly one of example_value_a or example_value_b must be provided."
+  }
+}
+				`,
+				ConfigVariables: config.Variables{
+					"example_value_b": config.StringVariable("example-value"),
+				},
+				Check: resource.ComposeAggregateTestCheckFunc(),
+			},
+		},
+	})
+}
+
+func TestNullFunction_falseCases(t *testing.T) {
 	t.Parallel()
 	resource.UnitTest(t, resource.TestCase{
 		TerraformVersionChecks: []tfversion.TerraformVersionCheck{