diff --git a/libtests/arg_parser.cc b/libtests/arg_parser.cc
index 0c7e6787..4b10709e 100644
--- a/libtests/arg_parser.cc
+++ b/libtests/arg_parser.cc
@@ -3,6 +3,11 @@
 #include <qpdf/QPDFUsage.hh>
 #include <iostream>
 #include <cstring>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
 #include <cassert>
 
 class ArgParser
diff --git a/libtests/buffer.cc b/libtests/buffer.cc
index ea36a807..5dfe55f8 100644
--- a/libtests/buffer.cc
+++ b/libtests/buffer.cc
@@ -5,9 +5,14 @@
 #include <stdlib.h>
 #include <stdexcept>
 #include <iostream>
-#include <cassert>
 #include <cstring>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 static unsigned char* uc(char const* s)
 {
     return QUtil::unsigned_char_pointer(s);
diff --git a/libtests/concatenate.cc b/libtests/concatenate.cc
index 16850d03..84fee2bb 100644
--- a/libtests/concatenate.cc
+++ b/libtests/concatenate.cc
@@ -3,7 +3,12 @@
 #include <qpdf/Pl_Buffer.hh>
 #include <qpdf/QUtil.hh>
 #include <iostream>
-#include <assert.h>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
 
 static void pipeStringAndFinish(Pipeline* p, std::string const& str)
 {
diff --git a/libtests/cxx11.cc b/libtests/cxx11.cc
index bc14b5c0..5ad3e6ed 100644
--- a/libtests/cxx11.cc
+++ b/libtests/cxx11.cc
@@ -1,5 +1,4 @@
 #include <iostream>
-#include <cassert>
 #include <cstring>
 #include <cstdlib>
 #include <functional>
@@ -10,6 +9,12 @@
 #include <memory>
 #include <regex>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 // Functional programming
 
 // Function that returns a callable in the form of a lambda
diff --git a/libtests/json.cc b/libtests/json.cc
index c37af0e8..a8c72a3b 100644
--- a/libtests/json.cc
+++ b/libtests/json.cc
@@ -1,6 +1,11 @@
 #include <qpdf/JSON.hh>
 #include <qpdf/QPDFObjectHandle.hh>
 #include <iostream>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
 #include <cassert>
 
 static void check(JSON const& j, std::string const& exp)
diff --git a/libtests/json_handler.cc b/libtests/json_handler.cc
index 568677a8..6e894e11 100644
--- a/libtests/json_handler.cc
+++ b/libtests/json_handler.cc
@@ -2,6 +2,11 @@
 #include <qpdf/QUtil.hh>
 #include <qpdf/QPDFUsage.hh>
 #include <iostream>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
 #include <cassert>
 
 static void print_null(std::string const& path)
diff --git a/libtests/matrix.cc b/libtests/matrix.cc
index ecaa02a0..210c022f 100644
--- a/libtests/matrix.cc
+++ b/libtests/matrix.cc
@@ -1,8 +1,13 @@
 #include <qpdf/QPDFMatrix.hh>
 #include <qpdf/QUtil.hh>
-#include <assert.h>
 #include <iostream>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 static void check(QPDFMatrix const& m, std::string const& exp)
 {
     std::string u = m.unparse();
diff --git a/libtests/pdf_version.cc b/libtests/pdf_version.cc
index c8a568ed..2471cb0b 100644
--- a/libtests/pdf_version.cc
+++ b/libtests/pdf_version.cc
@@ -1,8 +1,13 @@
 #include <qpdf/PDFVersion.hh>
 
-#include <cassert>
 #include <iostream>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 int main()
 {
     PDFVersion v1;
diff --git a/libtests/predictors.cc b/libtests/predictors.cc
index d2c4863e..f9fa759e 100644
--- a/libtests/predictors.cc
+++ b/libtests/predictors.cc
@@ -6,10 +6,15 @@
 
 #include <iostream>
 #include <errno.h>
-#include <assert.h>
 #include <string.h>
 #include <stdlib.h>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 void run(char const* filename, char const* filter,
          bool encode, unsigned int columns,
          unsigned int bits_per_sample, unsigned int samples_per_pixel)
diff --git a/libtests/qintc.cc b/libtests/qintc.cc
index f9ca9558..85806d1a 100644
--- a/libtests/qintc.cc
+++ b/libtests/qintc.cc
@@ -1,5 +1,10 @@
 #include <qpdf/QIntC.hh>
 #include <stdint.h>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
 #include <cassert>
 
 #define try_convert(exp_pass, fn, i) \
diff --git a/libtests/qutil.cc b/libtests/qutil.cc
index b9b7c674..c3a9b163 100644
--- a/libtests/qutil.cc
+++ b/libtests/qutil.cc
@@ -9,7 +9,6 @@
 #include <qpdf/Pl_Buffer.hh>
 #include <string.h>
 #include <limits.h>
-#include <assert.h>
 #include <fstream>
 #include <locale>
 
@@ -19,6 +18,12 @@
 # include <unistd.h>
 #endif
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 template <class int_T>
 void test_to_number(char const* str, int_T wanted, bool error,
                     int_T (*fn)(char const*))
diff --git a/libtests/rc4.cc b/libtests/rc4.cc
index eab3a759..b7aecd4b 100644
--- a/libtests/rc4.cc
+++ b/libtests/rc4.cc
@@ -7,6 +7,11 @@
 #include <string.h>
 #include <iostream>
 #include <stdlib.h>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
 #include <cassert>
 
 static void other_tests()
diff --git a/libtests/sparse_array.cc b/libtests/sparse_array.cc
index 9c3e00b4..5636abd1 100644
--- a/libtests/sparse_array.cc
+++ b/libtests/sparse_array.cc
@@ -1,7 +1,12 @@
 #include <qpdf/SparseOHArray.hh>
-#include <assert.h>
 #include <iostream>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 int main()
 {
     SparseOHArray a;
diff --git a/qpdf/pdf_from_scratch.cc b/qpdf/pdf_from_scratch.cc
index 07170347..0dd5bbf7 100644
--- a/qpdf/pdf_from_scratch.cc
+++ b/qpdf/pdf_from_scratch.cc
@@ -9,7 +9,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
-#include <assert.h>
 
 static char const* whoami = 0;
 
diff --git a/qpdf/qpdf-ctest.c b/qpdf/qpdf-ctest.c
index dff7c8c4..3a890a69 100644
--- a/qpdf/qpdf-ctest.c
+++ b/qpdf/qpdf-ctest.c
@@ -1,12 +1,17 @@
 #include <qpdf/qpdf-c.h>
 #include <stdio.h>
-#include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
 
 #include <qpdf/qpdf-config.h> // for LL_FMT -- special case in build
 
+#ifdef NDEBUG
+/* We need assert even in a release build for test code. */
+# undef NDEBUG
+#endif
+#include <assert.h>
+
 static char* whoami = 0;
 static qpdf_data qpdf = 0;
 
diff --git a/qpdf/qpdfjob-ctest.c b/qpdf/qpdfjob-ctest.c
index 43935333..5ee21f31 100644
--- a/qpdf/qpdfjob-ctest.c
+++ b/qpdf/qpdfjob-ctest.c
@@ -1,9 +1,14 @@
 #include <qpdf/qpdfjob-c.h>
 #include <stdio.h>
-#include <assert.h>
 #include <stdlib.h>
 #include <string.h>
 
+#ifdef NDEBUG
+/* We need assert even in a release build for test code. */
+# undef NDEBUG
+#endif
+#include <assert.h>
+
 #ifndef QPDF_NO_WCHAR_T
 static void wide_test()
 {
diff --git a/qpdf/test_driver.cc b/qpdf/test_driver.cc
index aee3cd53..fd0733fc 100644
--- a/qpdf/test_driver.cc
+++ b/qpdf/test_driver.cc
@@ -28,10 +28,15 @@
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
-#include <assert.h>
 #include <limits.h>
 #include <map>
 
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
+
 static char const* whoami = 0;
 
 void usage()
diff --git a/qpdf/test_large_file.cc b/qpdf/test_large_file.cc
index 47796ba5..e403dd51 100644
--- a/qpdf/test_large_file.cc
+++ b/qpdf/test_large_file.cc
@@ -13,7 +13,12 @@
 #include <iostream>
 #include <string.h>
 #include <stdlib.h>
-#include <assert.h>
+
+#ifdef NDEBUG
+// We need assert even in a release build for test code.
+# undef NDEBUG
+#endif
+#include <cassert>
 
 // Run "test_large_file write small a.pdf" to get a PDF file that you
 // can look at in a reader.