diff --git a/mypy/report.py b/mypy/report.py
index 3e49ed29a47c..b5d37b128b37 100644
--- a/mypy/report.py
+++ b/mypy/report.py
@@ -295,7 +295,20 @@ def indentation_level(self, line_number: int) -> Optional[int]:
 
     def visit_func_def(self, defn: FuncDef) -> None:
         start_line = defn.get_line() - 1
-        start_indent = self.indentation_level(start_line)
+        start_indent = None
+        # When a function is decorated, sometimes the start line will point to
+        # whitespace or comments between the decorator and the function, so
+        # we have to look for the start.
+        while start_line < len(self.source):
+            start_indent = self.indentation_level(start_line)
+            if start_indent is not None:
+                break
+            start_line += 1
+        # If we can't find the function give up and don't annotate anything.
+        # Our line numbers are not reliable enough to be asserting on.
+        if start_indent is None:
+            return
+
         cur_line = start_line + 1
         end_line = cur_line
         # After this loop, function body will be lines [start_line, end_line)
@@ -315,8 +328,12 @@ def visit_func_def(self, defn: FuncDef) -> None:
         is_typed = defn.type is not None
         for line in range(start_line, end_line):
             old_indent, _ = self.lines_covered[line]
-            assert start_indent is not None and start_indent > old_indent
-            self.lines_covered[line] = (start_indent, is_typed)
+            # If there was an old indent level for this line, and the new
+            # level isn't increasing the indentation, ignore it.
+            # This is to be defensive against funniness in our line numbers,
+            # which are not always reliable.
+            if old_indent <= start_indent:
+                self.lines_covered[line] = (start_indent, is_typed)
 
         # Visit the body, in case there are nested functions
         super().visit_func_def(defn)
diff --git a/test-data/unit/reports.test b/test-data/unit/reports.test
index bb0f4f55c7c9..11a4dc060880 100644
--- a/test-data/unit/reports.test
+++ b/test-data/unit/reports.test
@@ -337,3 +337,23 @@ def foo():
     n             0          0            0                  0       0              0                         0
 -----------------------------------------------------------------------------------------------------------------
 Total             0          0            0                  0       0              0                         0
+[case testTrickyCoverage]
+# cmd: mypy --linecoverage-report=report n.py
+[file n.py]
+import attr
+
+def blah(x): return x
+
+@blah
+
+def f(x: int) -> None: pass
+
+class Foo:
+    @blah
+    #hi
+    def f(self, x: int) -> None:
+        pass
+
+@attr.s
+class Z(object):
+    pass