diff --git a/lib/rbs/unit_test/type_assertions.rb b/lib/rbs/unit_test/type_assertions.rb
index 6794218a29..686fe1aea6 100644
--- a/lib/rbs/unit_test/type_assertions.rb
+++ b/lib/rbs/unit_test/type_assertions.rb
@@ -180,11 +180,11 @@ def send_setup(method_type, receiver, method, args, proc)
           )
           errors = typecheck.method_call(method, method_type, trace, errors: [])
 
-          assert_empty errors.map {|x| RBS::Test::Errors.to_string(x) }, "Call trace does not match with given method type: #{trace.inspect}"
+          assert_empty errors.map {|x| RBS::Test::Errors.to_string(x) }, -> { "Call trace does not match with given method type: #{trace.inspect}" }
 
           method_defs = method_defs(method)
           all_errors = method_defs.map {|t| typecheck.method_call(method, t.type, trace, errors: [], annotations: t.each_annotation.to_a) }
-          assert all_errors.any? {|es| es.empty? }, "Call trace does not match one of method definitions:\n  #{trace.inspect}\n  #{method_defs.map(&:type).join(" | ")}"
+          assert all_errors.any? {|es| es.empty? }, -> { "Call trace does not match one of method definitions:\n  #{trace.inspect}\n  #{method_defs.map(&:type).join(" | ")}" }
 
           raise exception if exception
 
@@ -204,11 +204,11 @@ def send_setup(method_type, receiver, method, args, proc)
           )
           errors = typecheck.method_call(method, method_type, trace, errors: [])
 
-          assert_empty errors.map {|x| RBS::Test::Errors.to_string(x) }, "Call trace does not match with given method type: #{trace.inspect}"
+          assert_empty errors.map {|x| RBS::Test::Errors.to_string(x) }, -> { "Call trace does not match with given method type: #{trace.inspect}" }
 
           method_defs = method_defs(method)
           all_errors = method_defs.map {|t| typecheck.method_call(method, t.type, trace, errors: [], annotations: t.each_annotation.to_a) }
-          assert all_errors.any? {|es| es.empty? }, "Call trace does not match one of method definitions:\n  #{trace.inspect}\n  #{method_defs.map(&:type).join(" | ")}"
+          assert all_errors.any? {|es| es.empty? }, -> { "Call trace does not match one of method definitions:\n  #{trace.inspect}\n  #{method_defs.map(&:type).join(" | ")}" }
 
           # Use `instnace_of?` instead of `is_a?` as we want to check for _the exact exception class_.
           assert exception.instance_of? error_type
@@ -246,7 +246,7 @@ def send_setup(method_type, receiver, method, args, proc)
 
           method_defs = method_defs(method)
           all_errors = method_defs.map {|t| typecheck.method_call(method, t.type, trace, errors: [], annotations: t.each_annotation.to_a) }
-          assert all_errors.all? {|es| es.size > 0 }, "Call trace unexpectedly matches one of method definitions:\n  #{trace.inspect}\n  #{method_defs.map(&:type).join(" | ")}"
+          assert all_errors.all? {|es| es.size > 0 }, -> { "Call trace unexpectedly matches one of method definitions:\n  #{trace.inspect}\n  #{method_defs.map(&:type).join(" | ")}" }
 
           result
         end
diff --git a/sig/unit_test/type_assertions.rbs b/sig/unit_test/type_assertions.rbs
index 5ef4396d60..98fb775b53 100644
--- a/sig/unit_test/type_assertions.rbs
+++ b/sig/unit_test/type_assertions.rbs
@@ -30,11 +30,11 @@ module RBS
       type target_type = Types::ClassInstance | Types::ClassSingleton
 
       interface _BaseAssertions
-        def assert: (untyped, ?String?) -> void
+        def assert: (untyped, ?(String | ^() -> String)?) -> void
 
         def refute: (untyped, ?String?) -> void
 
-        def assert_empty: (untyped, ?String?) -> void
+        def assert_empty: (untyped, ?(String | ^() -> String)?) -> void
 
         def assert_operator: (untyped, Symbol, *untyped) -> void