diff --git a/line_profiler/_line_profiler.pyx b/line_profiler/_line_profiler.pyx
index 15510317..a6bb5a20 100644
--- a/line_profiler/_line_profiler.pyx
+++ b/line_profiler/_line_profiler.pyx
@@ -471,7 +471,19 @@ cdef class LineProfiler:
 
     @property
     def c_last_time(self):
-        return (<dict>self._c_last_time)[threading.get_ident()]
+        """
+        Raises:
+            KeyError
+                If no profiling data is available on the current thread.
+        """
+        try:
+            return (<dict>self._c_last_time)[threading.get_ident()]
+        except KeyError as e:
+            # We haven't actually profiled anything yet
+            raise (KeyError('No profiling data on the current thread '
+                            '(`threading.get_ident()` = '
+                            f'{threading.get_ident()})')
+                   .with_traceback(e.__traceback__)) from None
 
     @property
     def code_map(self):
@@ -500,13 +512,12 @@ cdef class LineProfiler:
         line_profiler 4.0 no longer directly maintains last_time, but this will
         construct something similar for backwards compatibility.
         """
-        c_last_time = (<dict>self._c_last_time)[threading.get_ident()]
-        code_hash_map = self.code_hash_map
+        c_last_time = self.c_last_time
         py_last_time = {}
-        for code, code_hashes in code_hash_map.items():
-            for code_hash in code_hashes:
-                if code_hash in c_last_time:
-                    py_last_time[code] = c_last_time[code_hash]
+        for code in self.code_hash_map:
+            block_hash = hash(code.co_code)
+            if block_hash in c_last_time:
+                py_last_time[code] = c_last_time[block_hash]
         return py_last_time
 
 
diff --git a/tests/test_line_profiler.py b/tests/test_line_profiler.py
index 954c77b1..8e15ff7b 100644
--- a/tests/test_line_profiler.py
+++ b/tests/test_line_profiler.py
@@ -80,6 +80,39 @@ def test_init():
     }
 
 
+def test_last_time():
+    """
+    Test that `LineProfiler.c_last_time` and `LineProfiler.last_time`
+    are consistent.
+    """
+    prof = LineProfiler()
+    with pytest.raises(KeyError, match='[Nn]o profiling data'):
+        prof.c_last_time
+
+    def get_last_time(prof, *, c=False):
+        try:
+            return getattr(prof, 'c_last_time' if c else 'last_time')
+        except KeyError:
+            return {}
+
+    @prof
+    def func():
+        return (get_last_time(prof, c=True).copy(),
+                get_last_time(prof).copy())
+
+    # These are always empty outside a profiling context
+    # (hence the need of the above function to capture the transient
+    # values)
+    assert not get_last_time(prof, c=True)
+    assert not get_last_time(prof)
+    # Inside `func()`, both should get an entry therefor
+    clt, lt = func()
+    assert not get_last_time(prof, c=True)
+    assert not get_last_time(prof)
+    assert set(clt) == {hash(func.__wrapped__.__code__.co_code)}
+    assert set(lt) == {func.__wrapped__.__code__}
+
+
 def test_enable_disable():
     lp = LineProfiler()
     assert lp.enable_count == 0