diff --git a/clang/lib/Format/UnwrappedLineFormatter.cpp b/clang/lib/Format/UnwrappedLineFormatter.cpp
index 8f0b05ecb91d6..0dcf3b7c8dd06 100644
--- a/clang/lib/Format/UnwrappedLineFormatter.cpp
+++ b/clang/lib/Format/UnwrappedLineFormatter.cpp
@@ -386,11 +386,14 @@ class LineJoiner {
           // Reduce indent level for bodies of namespaces which were compacted,
           // but only if their content was indented in the first place.
           auto *ClosingLine = AnnotatedLines.begin() + ClosingLineIndex + 1;
-          auto OutdentBy = I[J]->Level - TheLine->Level;
+          const int OutdentBy = I[J]->Level - TheLine->Level;
+          assert(OutdentBy >= 0);
           for (auto *CompactedLine = I + J; CompactedLine <= ClosingLine;
                ++CompactedLine) {
-            if (!(*CompactedLine)->InPPDirective)
-              (*CompactedLine)->Level -= OutdentBy;
+            if (!(*CompactedLine)->InPPDirective) {
+              const int Level = (*CompactedLine)->Level;
+              (*CompactedLine)->Level = std::max(Level - OutdentBy, 0);
+            }
           }
         }
         return J - 1;
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 21b681e290522..721cf29e0db47 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -4180,6 +4180,16 @@ TEST_F(FormatTest, FormatsNamespaces) {
                "void foo() {}\n"
                "} // namespace ns",
                Style);
+
+  FormatStyle LLVMWithCompactInnerNamespace = getLLVMStyle();
+  LLVMWithCompactInnerNamespace.CompactNamespaces = true;
+  LLVMWithCompactInnerNamespace.NamespaceIndentation = FormatStyle::NI_Inner;
+  verifyFormat("namespace ns1 { namespace ns2 { namespace ns3 {\n"
+               "// block for debug mode\n"
+               "#ifndef NDEBUG\n"
+               "#endif\n"
+               "}}} // namespace ns1::ns2::ns3",
+               LLVMWithCompactInnerNamespace);
 }
 
 TEST_F(FormatTest, NamespaceMacros) {