diff --git a/src/coreclr/debug/daccess/request.cpp b/src/coreclr/debug/daccess/request.cpp
index 1ebbdf40d9acf2..5bc105a14f814c 100644
--- a/src/coreclr/debug/daccess/request.cpp
+++ b/src/coreclr/debug/daccess/request.cpp
@@ -1613,11 +1613,8 @@ ClrDataAccess::GetObjectStringData(CLRDATA_ADDRESS obj, unsigned int count, _Ino
                 stringData[0] = W('\0');
             }
         }
-        else
-        {
-            hr = E_INVALIDARG;
-        }
 
+        // A size-only query (no output buffer) reports the needed size via pNeeded and succeeds.
         if (pNeeded)
             *pNeeded = needed;
     }
diff --git a/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/SOSDacImpl.cs b/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/SOSDacImpl.cs
index 2007bb81728a1e..08b0eb2b382acd 100644
--- a/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/SOSDacImpl.cs
+++ b/src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Legacy/SOSDacImpl.cs
@@ -3456,17 +3456,25 @@ int ISOSDacInterface.GetObjectStringData(ClrDataAddress obj, uint count, char* s
         if (_legacyImpl is not null)
         {
             char[] stringDataLocal = new char[count];
-            uint neededLocal;
+            uint neededLocal = 0;
             int hrLocal;
             fixed (char* ptr = stringDataLocal)
             {
-                hrLocal = _legacyImpl.GetObjectStringData(obj, count, ptr, &neededLocal);
+                // Invoke the legacy DAC under the same argument contract the caller gave the cDAC:
+                // only pass an output buffer when the caller did, and only request the size-out when
+                // the caller did. This keeps the HRESULT comparison apples-to-apples.
+                char* stringDataArg = stringData is null ? null : ptr;
+                uint* pNeededArg = pNeeded is null ? null : &neededLocal;
+                hrLocal = _legacyImpl.GetObjectStringData(obj, count, stringDataArg, pNeededArg);
             }
+
             Debug.ValidateHResult(hr, hrLocal);
             if (hr == HResults.S_OK)
             {
                 Debug.Assert(pNeeded == null || *pNeeded == neededLocal);
-                Debug.Assert(stringData == null || new ReadOnlySpan<char>(stringDataLocal, 0, (int)neededLocal - 1).SequenceEqual(new string(stringData)));
+                // Compare against the legacy buffer using the cDAC string length: neededLocal is only
+                // populated when a size-out was requested from the legacy DAC (mirroring the caller).
+                Debug.Assert(stringData == null || new ReadOnlySpan<char>(stringDataLocal, 0, new string(stringData).Length).SequenceEqual(new string(stringData)));
             }
         }
 #endif