diff --git a/contrib/seekable_format/tests/seekable_tests.c b/contrib/seekable_format/tests/seekable_tests.c
index 809ea6425eb..2c014b73eda 100644
--- a/contrib/seekable_format/tests/seekable_tests.c
+++ b/contrib/seekable_format/tests/seekable_tests.c
@@ -45,6 +45,7 @@ static int seekBuffWithTotal(void* opaque, long long offset, int origin)
         break;
     default:
         assert(0);  /* not possible */
+        return -1;  /* keeps newOffset from being read uninitialised when NDEBUG strips the assert */
     }
     if (newOffset > buff->size) {
         return -1;
diff --git a/contrib/seekable_format/zstdseek_decompress.c b/contrib/seekable_format/zstdseek_decompress.c
index ab9088a1061..7de90903a73 100644
--- a/contrib/seekable_format/zstdseek_decompress.c
+++ b/contrib/seekable_format/zstdseek_decompress.c
@@ -186,6 +186,7 @@ static int ZSTD_seekable_seek_buff(void* opaque, long long offset, int origin)
         break;
     default:
         assert(0);  /* not possible */
+        return -1;  /* keeps newOffset from being read uninitialised when NDEBUG strips the assert */
     }
     if (newOffset > buff->size) {
         return -1;