diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb92e86..cce65c2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 
 #### Fixes
 
+* [#399](https://github.com/ruby-grape/grape-entity/pull/399): Fix `ArgumentError` for single-argument lambdas in `expose` blocks and `if:` conditions - [@numbata](https://github.com/numbata).
 * Your contribution here.
 
 ### 1.0.2 (2026-04-13)
diff --git a/lib/grape_entity/entity.rb b/lib/grape_entity/entity.rb
index b80bc55..2951b0f 100644
--- a/lib/grape_entity/entity.rb
+++ b/lib/grape_entity/entity.rb
@@ -525,13 +525,10 @@ def serializable_hash(runtime_options = {})
     end
 
     def exec_with_object(options, &block)
-      arity = if symbol_to_proc_wrapper?(block)
-                ensure_block_arity!(block)
-              else
-                block.arity
-              end
-
-      if arity.zero?
+      if symbol_to_proc_wrapper?(block)
+        ensure_block_arity!(block)
+        instance_exec(object, &block)
+      elsif block.arity == 1
         instance_exec(object, &block)
       else
         instance_exec(object, options, &block)
@@ -542,7 +539,7 @@ def ensure_block_arity!(block)
       # MRI currently always includes "( &:foo )" for symbol-to-proc wrappers.
       # If this format changes in a new Ruby version, this logic must be updated.
       origin_method_name = block.to_s.scan(/(?<=\(&:)[^)]+(?=\))/).first&.to_sym
-      return 0 unless origin_method_name
+      return unless origin_method_name
 
       unless object.respond_to?(origin_method_name, true)
         raise ArgumentError, <<~MSG
@@ -551,7 +548,7 @@ def ensure_block_arity!(block)
       end
 
       arity = object.method(origin_method_name).arity
-      return 0 if arity.zero?
+      return if arity.zero?
 
       raise ArgumentError, <<~MSG
         Cannot use `&:#{origin_method_name}` because that method expects #{arity} argument#{'s' if arity != 1}.
diff --git a/spec/grape_entity/entity_spec.rb b/spec/grape_entity/entity_spec.rb
index 0ee975b..f024217 100644
--- a/spec/grape_entity/entity_spec.rb
+++ b/spec/grape_entity/entity_spec.rb
@@ -27,6 +27,11 @@
           expect { subject.expose :name, as: :foo }.not_to raise_error
         end
 
+        it 'supports callable :as with single-argument lambda' do
+          subject.expose :name, as: ->(obj) { obj[:name].upcase }
+          expect(subject.represent({ name: 'test' }).serializable_hash).to eq('TEST' => 'test')
+        end
+
         it 'makes sure that :format_with as a proc cannot be used with a block' do
           # rubocop:disable Style/BlockDelimiters
           expect {
@@ -148,6 +153,12 @@ def initialize(a, b, c)
               subject.expose(:c)
               expect(subject.represent(model, option_a: 100).serializable_hash).to eq(a: 100, b: nil, c: 'value')
             end
+
+            it 'works with single-argument block' do
+              subject.expose(:a, expose_nil: false) { |obj| obj.c }
+              subject.expose(:b)
+              expect(subject.represent(model).serializable_hash).to eq(a: 'value', b: nil)
+            end
           end
         end
 
@@ -476,6 +487,42 @@ def raises_argument_error
               end.to raise_error ArgumentError, match(/method is not defined in the object/)
             end
           end
+
+          context 'with single-argument block' do
+            it 'passes only the object without raising ArgumentError' do
+              subject.expose :that_method_without_args do |object|
+                object.method_without_args
+              end
+
+              object = SomeObject.new
+              value = subject.represent(object).value_for(:that_method_without_args)
+              expect(value).to eq('result')
+            end
+          end
+
+          context 'with two-argument block' do
+            it 'passes the object and options without raising ArgumentError' do
+              subject.expose :that_method_without_args do |object, _options|
+                object.method_without_args
+              end
+
+              object = SomeObject.new
+              value = subject.represent(object).value_for(:that_method_without_args)
+              expect(value).to eq('result')
+            end
+          end
+
+          context 'with splat-argument block' do
+            it 'passes the object and options' do
+              subject.expose :args_count do |*args|
+                args.size
+              end
+
+              object = SomeObject.new
+              value = subject.represent(object).value_for(:args_count)
+              expect(value).to eq(2)
+            end
+          end
         end
 
         context 'with no parameters passed to the block' do
@@ -521,6 +568,32 @@ def raises_argument_error
             expect(subject.represent({}).value_for(:awesome)).to eq(condition_met: 'value')
           end
 
+          it 'works with single-argument if condition lambdas' do
+            subject.expose :awesome do
+              subject.expose(:condition_met, if: ->(_) { true }) { |_| 'value' }
+              subject.expose(:condition_not_met, if: ->(_) { false }) { |_| 'value' }
+            end
+
+            expect(subject.represent({}).value_for(:awesome)).to eq(condition_met: 'value')
+          end
+
+          it 'works with two-argument if condition lambdas' do
+            subject.expose :awesome do
+              subject.expose(:condition_met, if: ->(_, _) { true }) { |_| 'value' }
+              subject.expose(:condition_not_met, if: ->(_, _) { false }) { |_| 'value' }
+            end
+
+            expect(subject.represent({}).value_for(:awesome)).to eq(condition_met: 'value')
+          end
+
+          it 'works with single-argument block exposures' do
+            subject.expose :awesome do
+              subject.expose(:nested) { |obj| obj.class.name }
+            end
+
+            expect(subject.represent({}).value_for(:awesome)).to eq(nested: 'Hash')
+          end
+
           it 'does not represent attributes, declared inside nested exposure, outside of it' do
             subject.expose :awesome do
               subject.expose(:nested) { |_| 'value' }