diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -142,6 +142,8 @@
     git diff | tee ${BUILD_DIR}/generated_output.patch
     # Check if the diffs are empty, fail otherwise.
     ! grep -q '^--- a' ${BUILD_DIR}/generated_output.patch
+    # Check that no dependency cycles have been introduced.
+    python3 libcxx/utils/graph_header_deps.py >/dev/null
 ;;
 generic-cxx03)
     export CC=clang
diff --git a/libcxx/utils/graph_header_deps.py b/libcxx/utils/graph_header_deps.py
--- a/libcxx/utils/graph_header_deps.py
+++ b/libcxx/utils/graph_header_deps.py
@@ -61,14 +61,18 @@
     system_includes = []
     linecount = 0
     with open(fname, 'r') as f:
-        for line in f.readlines():
-            linecount += 1
-            m = re.match(r'\s*#\s*include\s+"([^"]*)"', line)
-            if m is not None:
-                local_includes.append(m.group(1))
-            m = re.match(r'\s*#\s*include\s+<([^>]*)>', line)
-            if m is not None:
-                system_includes.append(m.group(1))
+        try:
+            for line in f.readlines():
+                linecount += 1
+                m = re.match(r'\s*#\s*include\s+"([^"]*)"', line)
+                if m is not None:
+                    local_includes.append(m.group(1))
+                m = re.match(r'\s*#\s*include\s+<([^>]*)>', line)
+                if m is not None:
+                    system_includes.append(m.group(1))
+        except UnicodeDecodeError as e:
+            sys.stderr.write('Non-ASCII detected in %s\n' % fname)
+            raise
 
     fully_qualified_includes = [
         locate_header_file(h, options.search_dirs)