diff --git a/src/runner.c b/src/runner.c
index de6c3987c0bd8e16dc99def6bb2fa9c1ff3d12e1..503cdd0e062afd732560612b6d6c6c7df593690f 100644
--- a/src/runner.c
+++ b/src/runner.c
@@ -54,6 +54,9 @@
 #define cell_getid(cdim, i, j, k) \
   ((int)(k) + (cdim)[2] * ((int)(j) + (cdim)[1] * (int)(i)))
 
+/* Histograms bins. */
+long long int runner_hist_bins[runner_hist_N];
+
 /* The counters. */
 int runner_counter[runner_counter_count];
 
diff --git a/src/runner.h b/src/runner.h
index 1ad348ccceea1549fe0d04bf7cac06dfed9b19bc..02b1f1f753eb84c248c83e714577eb4c3953be2d 100644
--- a/src/runner.h
+++ b/src/runner.h
@@ -58,7 +58,7 @@ extern int runner_counter[runner_counter_count];
 #define runner_hist_a 1.0
 #define runner_hist_b 100.0
 #define runner_hist_N 99
-long long int runner_hist_bins[runner_hist_N];
+extern long long int runner_hist_bins[runner_hist_N];
 #define runner_hist_hit(x)                                                   \
   __sync_add_and_fetch(                                                      \
       &runner_hist_bins[(int)fmax(                                           \