diff --git a/icu4c/source/tools/ctestfw/tstdtmod.cpp b/icu4c/source/tools/ctestfw/tstdtmod.cpp
index 5a8e1ef7365..15e8913d2b8 100644
--- a/icu4c/source/tools/ctestfw/tstdtmod.cpp
+++ b/icu4c/source/tools/ctestfw/tstdtmod.cpp
@@ -11,6 +11,7 @@
 #include <stdarg.h>
 
 #include "unicode/tstdtmod.h"
+#include "unicode/unistr.h"
 #include "cmemory.h"
 #include <stdio.h>
 #include "cstr.h"
@@ -18,11 +19,17 @@
 
 TestLog::~TestLog() {}
 
+IcuTestErrorCode::IcuTestErrorCode(TestLog &callingTestClass, const char *callingTestName)
+    : errorCode(U_ZERO_ERROR),
+      testClass(callingTestClass), testName(callingTestName), scopeMessage(*new UnicodeString) {
+}
+
 IcuTestErrorCode::~IcuTestErrorCode() {
     // Safe because our errlog() does not throw exceptions.
     if(isFailure()) {
         errlog(false, u"destructor: expected success", nullptr);
     }
+    delete &scopeMessage;
 }
 
 UErrorCode IcuTestErrorCode::reset() {
@@ -119,8 +126,7 @@ UBool IcuTestErrorCode::expectErrorAndReset(UErrorCode expectedError, const char
 }
 
 void IcuTestErrorCode::setScope(const char* message) {
-    UnicodeString us(message, -1, US_INV);
-    scopeMessage = us;
+    scopeMessage.remove().append({ message, -1, US_INV });
 }
 
 void IcuTestErrorCode::setScope(std::u16string_view message) {
@@ -136,7 +142,7 @@ void IcuTestErrorCode::errlog(UBool dataErr, std::u16string_view mainMessage, co
     msg.append(u' ').append(mainMessage);
     msg.append(u" but got error: ").append(UnicodeString(errorName(), -1, US_INV));
 
-    if (!scopeMessage.empty()) {
+    if (!scopeMessage.isEmpty()) {
         msg.append(u" scope: ").append(scopeMessage);
     }
 
diff --git a/icu4c/source/tools/ctestfw/unicode/testlog.h b/icu4c/source/tools/ctestfw/unicode/testlog.h
index a5e62e80d73..44e99954f91 100644
--- a/icu4c/source/tools/ctestfw/unicode/testlog.h
+++ b/icu4c/source/tools/ctestfw/unicode/testlog.h
@@ -13,7 +13,6 @@
 #ifndef U_TESTFW_TESTLOG
 #define U_TESTFW_TESTLOG
 
-#include <string>
 #include <string_view>
 #include "unicode/utypes.h"
 #include "unicode/testtype.h"
@@ -35,14 +34,16 @@ public:
 // unit tests that work without U_SHOW_CPLUSPLUS_API.
 // So instead we *copy* the ErrorCode API.
 
+U_NAMESPACE_BEGIN
+class UnicodeString;
+U_NAMESPACE_END
+
 class T_CTEST_EXPORT_API IcuTestErrorCode {
 public:
     IcuTestErrorCode(const IcuTestErrorCode&) = delete;
     IcuTestErrorCode& operator=(const IcuTestErrorCode&) = delete;
 
-    IcuTestErrorCode(TestLog &callingTestClass, const char *callingTestName)
-            : errorCode(U_ZERO_ERROR),
-              testClass(callingTestClass), testName(callingTestName), scopeMessage() {}
+    IcuTestErrorCode(TestLog &callingTestClass, const char *callingTestName);
     virtual ~IcuTestErrorCode();
 
     // ErrorCode API
@@ -75,7 +76,14 @@ private:
     UErrorCode errorCode;
     TestLog &testClass;
     const char *const testName;
-    std::u16string scopeMessage;
+
+    // It's not possible to use a UnicodeString member directly here because
+    // that won't work without U_SHOW_CPLUSPLUS_API, but it's also not possible
+    // to use a std::u16string member because for unknown reasons that leads to
+    // a crash in the icu4c-windows-cygwin-gcc CI job. As a workaround, the
+    // UnicodeString class is forward declared to make it possible to use a
+    // reference here and then heap allocate the object in the constructor.
+    UnicodeString& scopeMessage;
 
     void errlog(UBool dataErr, std::u16string_view mainMessage, const char* extraMessage) const;
 };