diff --git a/src/Network/Wai/Middleware/Prometheus.hs b/src/Network/Wai/Middleware/Prometheus.hs
--- a/src/Network/Wai/Middleware/Prometheus.hs
+++ b/src/Network/Wai/Middleware/Prometheus.hs
@@ -8,8 +8,11 @@
   , PrometheusSettings(..)
   , Default.def
   , instrumentHandlerValue
+  , instrumentHandlerValueWithFilter
+  , ignoreRawResponses
   , instrumentApp
   , instrumentIO
+  , observeSeconds
   , metricsApp
   ) where
 
@@ -21,6 +24,7 @@
 import Data.Text.Encoding (decodeUtf8)
 import qualified Network.HTTP.Types as HTTP
 import qualified Network.Wai as Wai
+import qualified Network.Wai.Internal as Wai (Response(ResponseRaw))
 import qualified Prometheus as Prom
 import System.Clock (Clock(..), TimeSpec, diffTimeSpec, getTime, toNanoSecs)
 
@@ -60,19 +64,55 @@
 -- If you use this function you will likely want to override the default value
 -- of 'prometheusInstrumentApp' to be false so that your app does not get double
 -- instrumented.
+--
+-- WARNING: If you have 'ResponseRaw' values in your API, consider using
+-- @instrumentHandlerValueWithFilter ignoreRawResponses@ instead.
 instrumentHandlerValue ::
      (Wai.Request -> Text) -- ^ The function used to derive the "handler" value in Prometheus
   -> Wai.Application -- ^ The app to instrument
   -> Wai.Application -- ^ The instrumented app
-instrumentHandlerValue f app req respond = do
+instrumentHandlerValue = instrumentHandlerValueWithFilter Just
+
+-- | A more flexible variant of 'instrumentHandlerValue'.  The filter can change some
+-- responses, or drop others entirely.
+instrumentHandlerValueWithFilter ::
+     (Wai.Response -> Maybe Wai.Response) -- ^ Response filter
+  -> (Wai.Request -> Text) -- ^ The function used to derive the "handler" value in Prometheus
+  -> Wai.Application -- ^ The app to instrument
+  -> Wai.Application -- ^ The instrumented app
+instrumentHandlerValueWithFilter resFilter f app req respond = do
   start <- getTime Monotonic
   app req $ \res -> do
-    end <- getTime Monotonic
-    let method = Just $ decodeUtf8 (Wai.requestMethod req)
-    let status = Just $ T.pack (show (HTTP.statusCode (Wai.responseStatus res)))
-    observeSeconds (f req) method status start end
+    case resFilter res of
+      Nothing -> return ()
+      Just res' -> do
+        end <- getTime Monotonic
+        let method = Just $ decodeUtf8 (Wai.requestMethod req)
+        let status = Just $ T.pack (show (HTTP.statusCode (Wai.responseStatus res')))
+        observeSeconds (f req) method status start end
     respond res
 
+-- | 'Wai.ResponseRaw' values have two parts: an action that can be executed to construct a
+-- 'Wai.Response', and a pure "backup" 'Wai.Response' in case the computation fails.  Since
+-- the pure selectors like 'Wai.responseStatus' are pure and it makes no sense for them to
+-- call the action, they just go to the backup response and pull the value from that:
+--
+-- @
+-- responseStatus (ResponseRaw ...
+--   (ResponseBuilder (Status 500 "blargh") ... ...))
+-- == Status {statusCode = 500, statusMessage = "blargh"}
+-- @
+--
+-- This is often not what you want.  For example, if you have an end-point for establishing
+-- websocket connections that has a backup response with status 5xx, every websocket
+-- connection request, whether successful or not, will register as an internal server error.
+--
+-- This helper therefore filters out all raw requests so they won't create any metrics.  Use
+-- together with 'instrumentHandlerValueWithFilter'.
+ignoreRawResponses :: Wai.Response -> Maybe Wai.Response
+ignoreRawResponses (Wai.ResponseRaw {}) = Nothing
+ignoreRawResponses res = Just res
+
 -- | Instrument a WAI app with the default WAI metrics.
 --
 -- If you use this function you will likely want to override the default value
@@ -102,9 +142,16 @@
     observeSeconds label Nothing Nothing start end
     return result
 
-observeSeconds :: Text -> Maybe Text -> Maybe Text -> TimeSpec -> TimeSpec -> IO ()
+-- | Record an event to the middleware metric.
+observeSeconds :: Text         -- ^ handler label
+               -> Maybe Text   -- ^ method
+               -> Maybe Text   -- ^ status
+               -> TimeSpec     -- ^ start time
+               -> TimeSpec     -- ^ end time
+               -> IO ()
 observeSeconds handler method status start end = do
-    let latency = fromRational $ toRational (toNanoSecs (end `diffTimeSpec` start) % 1000000000)
+    let latency :: Double
+        latency = fromRational $ toRational (toNanoSecs (end `diffTimeSpec` start) % 1000000000)
     Prom.withLabel requestLatency
                    (handler, fromMaybe "" method, fromMaybe "" status)
                    (flip Prom.observe latency)
diff --git a/wai-middleware-prometheus.cabal b/wai-middleware-prometheus.cabal
--- a/wai-middleware-prometheus.cabal
+++ b/wai-middleware-prometheus.cabal
@@ -1,5 +1,5 @@
 name:                wai-middleware-prometheus
-version:             1.0.0
+version:             1.0.0.1
 synopsis:
     WAI middlware for exposing http://prometheus.io metrics.
 description:
@@ -29,7 +29,7 @@
     , clock
     , data-default
     , http-types
-    , prometheus-client  >=1.0.0 && <1.1
+    , prometheus-client  >=1.0.0 && <1.2
     , text               >=0.11
     , wai                >=3.0
   ghc-options: -Wall