diff --git a/Misc/NEWS.d/next/Library/2026-07-07-13-31-52.gh-issue-148286.-qu-em.rst b/Misc/NEWS.d/next/Library/2026-07-07-13-31-52.gh-issue-148286.-qu-em.rst
new file mode 100644
index 000000000000000..60cd49020c954a4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-07-13-31-52.gh-issue-148286.-qu-em.rst
@@ -0,0 +1,3 @@
+Fix undefined behavior in
+:attr:`compression.zstd.ZstdDecompressor.unused_data` when a complete frame
+was decompressed in a single call.
diff --git a/Modules/_zstd/decompressor.c b/Modules/_zstd/decompressor.c
index c9b57a898e7903f..85b67d468864890 100644
--- a/Modules/_zstd/decompressor.c
+++ b/Modules/_zstd/decompressor.c
@@ -594,9 +594,14 @@ _zstd_ZstdDecompressor_unused_data_get_impl(ZstdDecompressor *self)
     }
     else {
         if (self->unused_data == NULL) {
-            self->unused_data = PyBytes_FromStringAndSize(
-                                    self->input_buffer + self->in_begin,
-                                    self->in_end - self->in_begin);
+            if (self->input_buffer == NULL) {
+                self->unused_data = Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
+            }
+            else {
+                self->unused_data = PyBytes_FromStringAndSize(
+                                        self->input_buffer + self->in_begin,
+                                        self->in_end - self->in_begin);
+            }
             ret = self->unused_data;
             Py_XINCREF(ret);
         }