diff --git a/jedi/api/classes.py b/jedi/api/classes.py
index 19be914a..52ead9f7 100644
--- a/jedi/api/classes.py
+++ b/jedi/api/classes.py
@@ -293,6 +293,39 @@ class BaseDefinition(object):
 
         return '.'.join(path if path[0] else path[1:])
 
+    def _is_callable(self):
+        stripped = self._definition
+        if isinstance(stripped, pr.Name):
+            stripped = stripped.parent
+
+        return (stripped.isinstance(er.Function, er.Class)
+                or stripped.isinstance(compiled.CompiledObject)
+                and stripped.type() in ('function', 'class'))
+
+    @property
+    def params(self):
+        """
+        Raises an ``AttributeError``if the definition is not callable.
+        Otherwise returns a list of `Definition` that represents the params.
+        """
+        if not self._is_callable():
+            raise AttributeError()
+
+        if self._definition.isinstance(er.Function):
+            if isinstance(self._definition, er.InstanceElement):
+                params = self._definition.params[1:]
+            else:
+                params = self._definition.params
+        elif self._definition.isinstance(er.compiled.CompiledObject):
+            params = self._definition.params
+        else:
+            try:
+                sub = self._definition.get_subscope_by_name('__init__')
+                params = sub.params[1:]  # ignore self
+            except KeyError:
+                return []
+        return [_Param(self._evaluator, p) for p in params]
+
     def __repr__(self):
         return "<%s %s>" % (type(self).__name__, self.description)
 
@@ -602,23 +635,6 @@ class CallSignature(Definition):
         """ The param index of the current call. """
         self._call = call
 
-    @property
-    def params(self):
-        if self._definition.isinstance(er.Function):
-            if isinstance(self._definition, er.InstanceElement):
-                params = self._definition.params[1:]
-            else:
-                params = self._definition.params
-        elif self._definition.isinstance(er.compiled.CompiledObject):
-            params = self._definition.params
-        else:
-            try:
-                sub = self._definition.get_subscope_by_name('__init__')
-                params = sub.params[1:]  # ignore self
-            except KeyError:
-                return []
-        return [_Param(self._evaluator, p) for p in params]
-
     @property
     def bracket_start(self):
         """
diff --git a/test/test_api/test_api_classes.py b/test/test_api/test_api_classes.py
index 171a425e..c334be93 100644
--- a/test/test_api/test_api_classes.py
+++ b/test/test_api/test_api_classes.py
@@ -1,7 +1,7 @@
 """ Test all things related to the ``jedi.api_classes`` module.
 """
 
-import textwrap
+from textwrap import dedent
 from inspect import cleandoc
 
 import pytest
@@ -23,7 +23,7 @@ def make_definitions():
 
     :rtype: [jedi.api_classes.BaseDefinition]
     """
-    source = textwrap.dedent("""
+    source = dedent("""
     import sys
 
     class C:
@@ -43,7 +43,7 @@ def make_definitions():
     definitions = []
     definitions += defined_names(source)
 
-    source += textwrap.dedent("""
+    source += dedent("""
     variable = sys or C or x or f or g or g() or h""")
     lines = source.splitlines()
     script = Script(source, len(lines), len('variable'), None)
@@ -123,3 +123,19 @@ def test_completion_documentation():
 
     c = Script('import jedi\njedi.Scr').completions()[0]
     assert c.documentation(fast=False).raw() == cleandoc(Script.__doc__)
+
+
+def test_signature_params():
+    s = dedent('''
+    def foo(bar):
+        pass
+    foo''')
+
+    defs = Script(s).goto_definitions()
+    params = defs[0].params
+    assert len(params) == 1
+    assert params[0].name == 'bar'
+
+    defs = Script(s).goto_assignments()
+    with pytest.raises(AttributeError):
+        params = defs[0].params