diff --git a/tests/src/unit-regression3.cpp b/tests/src/unit-regression3.cpp
index e9ae61a6a..c65321c84 100644
--- a/tests/src/unit-regression3.cpp
+++ b/tests/src/unit-regression3.cpp
@@ -27,6 +27,7 @@ using json = nlohmann::json;
 using ordered_json = nlohmann::ordered_json;
 
 #include <cmath>
+#include <cstddef>
 #include <cstdint>
 #include <limits>
 #include <string>
@@ -60,6 +61,12 @@ using ordered_json = nlohmann::ordered_json;
     >;
 #endif
 
+#ifdef JSON_HAS_CPP_20
+    #if __has_include(<span>)
+        #include <span>
+    #endif
+#endif
+
 #if JSON_HAS_RANGES == 1
     #include <ranges>
 #endif