diff --git a/bake/bake_test.go b/bake/bake_test.go
index d946cb1c..03bc532d 100644
--- a/bake/bake_test.go
+++ b/bake/bake_test.go
@@ -2142,6 +2142,73 @@ target "app" {
 	})
 }
 
+func TestVariableValidationConditionNull(t *testing.T) {
+	fp := File{
+		Name: "docker-bake.hcl",
+		Data: []byte(`
+variable "PORT" {
+  default = 3000
+  validation {}
+}
+target "app" {
+  args = {
+    PORT = PORT
+  }
+}
+`),
+	}
+
+	_, _, err := ReadTargets(context.TODO(), []File{fp}, []string{"app"}, nil, nil, &EntitlementConf{})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "Condition expression must return either true or false, not null")
+}
+
+func TestVariableValidationConditionUnknownValue(t *testing.T) {
+	fp := File{
+		Name: "docker-bake.hcl",
+		Data: []byte(`
+variable "PORT" {
+  default = 3000
+  validation {
+    condition = "foo"
+  }
+}
+target "app" {
+  args = {
+    PORT = PORT
+  }
+}
+`),
+	}
+
+	_, _, err := ReadTargets(context.TODO(), []File{fp}, []string{"app"}, nil, nil, &EntitlementConf{})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "Invalid condition result value: a bool is required")
+}
+
+func TestVariableValidationInvalidErrorMessage(t *testing.T) {
+	fp := File{
+		Name: "docker-bake.hcl",
+		Data: []byte(`
+variable "FOO" {
+  default = 0
+  validation {
+    condition = FOO > 5
+  }
+}
+target "app" {
+  args = {
+    FOO = FOO
+  }
+}
+`),
+	}
+
+	_, _, err := ReadTargets(context.TODO(), []File{fp}, []string{"app"}, nil, nil, &EntitlementConf{})
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "This check failed, but has an invalid error message")
+}
+
 // https://github.com/docker/buildx/issues/2822
 func TestVariableEmpty(t *testing.T) {
 	fp := File{
diff --git a/bake/hclparser/hclparser.go b/bake/hclparser/hclparser.go
index 6f98cfb6..6aa5eada 100644
--- a/bake/hclparser/hclparser.go
+++ b/bake/hclparser/hclparser.go
@@ -16,6 +16,7 @@ import (
 	"github.com/hashicorp/hcl/v2"
 	"github.com/pkg/errors"
 	"github.com/zclconf/go-cty/cty"
+	"github.com/zclconf/go-cty/cty/convert"
 )
 
 type Opt struct {
@@ -555,27 +556,57 @@ func (p *parser) resolveBlockNames(block *hcl.Block) ([]string, error) {
 func (p *parser) validateVariables(vars map[string]*variable, ectx *hcl.EvalContext) hcl.Diagnostics {
 	var diags hcl.Diagnostics
 	for _, v := range vars {
-		for _, validation := range v.Validations {
-			condition, condDiags := validation.Condition.Value(ectx)
+		for _, rule := range v.Validations {
+			resultVal, condDiags := rule.Condition.Value(ectx)
 			if condDiags.HasErrors() {
 				diags = append(diags, condDiags...)
 				continue
 			}
-			if !condition.True() {
-				message, msgDiags := validation.ErrorMessage.Value(ectx)
+
+			if resultVal.IsNull() {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity:   hcl.DiagError,
+					Summary:    "Invalid condition result",
+					Detail:     "Condition expression must return either true or false, not null.",
+					Subject:    rule.Condition.Range().Ptr(),
+					Expression: rule.Condition,
+				})
+				continue
+			}
+
+			var err error
+			resultVal, err = convert.Convert(resultVal, cty.Bool)
+			if err != nil {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity:   hcl.DiagError,
+					Summary:    "Invalid condition result",
+					Detail:     fmt.Sprintf("Invalid condition result value: %s", err),
+					Subject:    rule.Condition.Range().Ptr(),
+					Expression: rule.Condition,
+				})
+				continue
+			}
+
+			if !resultVal.True() {
+				message, msgDiags := rule.ErrorMessage.Value(ectx)
 				if msgDiags.HasErrors() {
 					diags = append(diags, msgDiags...)
 					continue
 				}
+				errorMessage := "This check failed, but has an invalid error message."
+				if !message.IsNull() {
+					errorMessage = message.AsString()
+				}
 				diags = append(diags, &hcl.Diagnostic{
 					Severity: hcl.DiagError,
 					Summary:  "Validation failed",
-					Detail:   message.AsString(),
-					Subject:  validation.Condition.Range().Ptr(),
+					Detail:   errorMessage,
+					Subject:  rule.Condition.Range().Ptr(),
 				})
 			}
 		}
 	}
+
 	return diags
 }