diff --git a/prometheus-client.cabal b/prometheus-client.cabal
--- a/prometheus-client.cabal
+++ b/prometheus-client.cabal
@@ -1,5 +1,5 @@
 name:                prometheus-client
-version:             1.0.0
+version:             1.0.0.1
 synopsis:            Haskell client library for http://prometheus.io.
 description:         Haskell client library for http://prometheus.io.
 homepage:            https://github.com/fimad/prometheus-haskell
diff --git a/src/Prometheus/Metric/Histogram.hs b/src/Prometheus/Metric/Histogram.hs
--- a/src/Prometheus/Metric/Histogram.hs
+++ b/src/Prometheus/Metric/Histogram.hs
@@ -59,7 +59,7 @@
     -- | Counts for each bucket. The key is the upper-bound,
     -- value is the number of observations less-than-or-equal-to
     -- that upper bound, but greater than the next lowest upper bound.
-,   histCountsPerBucket :: Map.Map Bucket Int
+,   histCountsPerBucket :: !(Map.Map Bucket Int)
 } deriving (Show, Eq, Ord)
 
 emptyCounts :: [Bucket] -> BucketCounts