diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2740593..63985df 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -291,10 +291,23 @@ elseif(LIBOWFAT_BUILD_STATIC)
   add_library(libowfat::libowfat ALIAS libowfat_static)
 endif()
 
+function(_libowfat_disable_ndebug target)
+  # A number of upstream unit tests use assert() for all checks and deliberately
+  # refuse to compile when NDEBUG is set by a Release build.  Compile tests with
+  # asserts enabled and without optimisation even when the library itself is built
+  # as Release, matching the intent of the original test programs.
+  if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
+    target_compile_options(${target} PRIVATE -UNDEBUG -O0)
+  elseif(MSVC)
+    target_compile_options(${target} PRIVATE /UNDEBUG)
+  endif()
+endfunction()
+
 
 if(LIBOWFAT_RUN_SELF_TESTS)
   add_executable(libowfat_smoke_t "${CMAKE_CURRENT_SOURCE_DIR}/t.c")
   target_link_libraries(libowfat_smoke_t PRIVATE libowfat::libowfat)
+  _libowfat_disable_ndebug(libowfat_smoke_t)
   add_test(NAME libowfat_smoke_t COMMAND libowfat_smoke_t)
   set_tests_properties(libowfat_smoke_t PROPERTIES WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}")
 endif()
@@ -306,6 +319,7 @@ if(LIBOWFAT_BUILD_TEST_PROGRAMS)
     string(MAKE_C_IDENTIFIER "libowfat_test_${_test_name}" _test_target)
     add_executable(${_test_target} "${_test_src}")
     target_link_libraries(${_test_target} PRIVATE libowfat::libowfat)
+    _libowfat_disable_ndebug(${_test_target})
     if(CMAKE_C_COMPILER_ID MATCHES "GNU|Clang")
       target_compile_options(${_test_target} PRIVATE
         -Wno-error=implicit-int