diff --git a/gpiozero/__init__.py b/gpiozero/__init__.py
index 1c88d6d..6ac96ea 100644
--- a/gpiozero/__init__.py
+++ b/gpiozero/__init__.py
@@ -27,7 +27,6 @@ from .exc import (
     OutputDeviceError,
     OutputDeviceBadValue,
     PinError,
-    PinFixedFunction,
     PinInvalidFunction,
     PinInvalidState,
     PinInvalidPull,
diff --git a/gpiozero/exc.py b/gpiozero/exc.py
index 6d0900a..de2a530 100644
--- a/gpiozero/exc.py
+++ b/gpiozero/exc.py
@@ -64,9 +64,6 @@ class OutputDeviceBadValue(OutputDeviceError, ValueError):
 class PinError(GPIOZeroError):
     "Base class for errors related to pin implementations"
 
-class PinFixedFunction(PinError, AttributeError):
-    "Error raised when attempting to change the function of a fixed type pin"
-
 class PinInvalidFunction(PinError, ValueError):
     "Error raised when attempting to change the function of a pin to an invalid value"
 
diff --git a/gpiozero/input_devices.py b/gpiozero/input_devices.py
index c114fbf..bafc67a 100644
--- a/gpiozero/input_devices.py
+++ b/gpiozero/input_devices.py
@@ -39,8 +39,7 @@ class InputDevice(GPIODevice):
     def __init__(self, pin=None, pull_up=False):
         super(InputDevice, self).__init__(pin)
         try:
-            if self.pin.function != 'input':
-                self.pin.function = 'input'
+            self.pin.function = 'input'
             pull = 'up' if pull_up else 'down'
             if self.pin.pull != pull:
                 self.pin.pull = pull
diff --git a/gpiozero/pins/__init__.py b/gpiozero/pins/__init__.py
index 1ec0f95..e753076 100644
--- a/gpiozero/pins/__init__.py
+++ b/gpiozero/pins/__init__.py
@@ -7,7 +7,7 @@ from __future__ import (
 str = type('')
 
 from ..exc import (
-    PinFixedFunction,
+    PinInvalidFunction,
     PinSetInput,
     PinFixedPull,
     PinPWMUnsupported,
@@ -30,12 +30,12 @@ class Pin(object):
     overridden:
 
     * :meth:`_get_function`
+    * :meth:`_set_function`
     * :meth:`_get_state`
 
     The following functions *may* be overridden if applicable:
 
     * :meth:`close`
-    * :meth:`_set_function`
     * :meth:`_set_state`
     * :meth:`_get_frequency`
     * :meth:`_set_frequency`
@@ -105,7 +105,9 @@ class Pin(object):
         return "input"
 
     def _set_function(self, value):
-        raise PinFixedFunction("Cannot set the function of pin %r" % self)
+        if value != "input":
+            raise PinInvalidFunction(
+                "Cannot set the function of pin %r to %s" % (self, value))
 
     function = property(
         lambda self: self._get_function(),
@@ -119,8 +121,7 @@ class Pin(object):
         With certain pin types (e.g. GPIO pins), this attribute can be changed
         to configure the function of a pin. If an invalid function is
         specified, for this attribute, :exc:`PinInvalidFunction` will be
-        raised. If this pin is fixed function and an attempt is made to set
-        this attribute, :exc:`PinFixedFunction` will be raised.
+        raised.
         """)
 
     def _get_state(self):