diff --git a/include/wx/string.h b/include/wx/string.h
index a648beb6d7..cb6cf460a0 100644
--- a/include/wx/string.h
+++ b/include/wx/string.h
@@ -431,7 +431,10 @@ private:
       size_t len;
 
       SubstrBufFromType(const T& data_, size_t len_)
-          : data(data_), len(len_) {}
+          : data(data_), len(len_)
+      {
+          wxASSERT_MSG( len != npos, "must have real length" );
+      }
   };
 
 #if wxUSE_UNICODE_UTF8
diff --git a/src/common/string.cpp b/src/common/string.cpp
index a287759822..e68c7f03c0 100644
--- a/src/common/string.cpp
+++ b/src/common/string.cpp
@@ -369,6 +369,9 @@ wxString::SubstrBufFromMB wxString::ConvertStr(const char *psz, size_t nLength,
         // UTF-8 sequence and psz may be invalid:
         if ( wxStringOperations::IsValidUtf8String(psz, nLength) )
         {
+            // we must pass the real string length to SubstrBufFromMB ctor
+            if ( nLength == npos )
+                nLength = psz ? strlen(psz) : 0;
             return SubstrBufFromMB(wxCharBuffer::CreateNonOwned(psz), nLength);
         }
         // else: do the roundtrip through wchar_t*