diff --git a/marko/inline_parser.py b/marko/inline_parser.py
index e4d5bd9..e73d7c6 100644
--- a/marko/inline_parser.py
+++ b/marko/inline_parser.py
@@ -437,6 +437,7 @@ def process_emphasis(
                 underscore_bottom = bottom
             if not d_closer.can_open:
                 delimiters.remove(d_closer)
+                cur = cur - 1 if cur > 0 else None
         cur = _next_closer(delimiters, cur)
     lower = stack_bottom + 1 if stack_bottom is not None else 0
     del delimiters[lower:]
diff --git a/tests/test_spec.py b/tests/test_spec.py
index 2e72f56..22749fa 100644
--- a/tests/test_spec.py
+++ b/tests/test_spec.py
@@ -90,6 +90,11 @@ def gfm() -> Markdown:
         "![foo\nbar\nbaz](/image.png)",
         '<p><img src="/image.png" alt="foo\nbar\nbaz" /></p>',
     ),
+    (
+        "emphasis_underscore_in_strong",
+        "**a_{b}**",
+        "<p><strong>a_{b}</strong></p>",
+    )
 ]