diff --git a/lib/IDS/DeReKoVecs/derekovecs-server.c b/lib/IDS/DeReKoVecs/derekovecs-server.c
index e4cc096..9d073a4 100644
--- a/lib/IDS/DeReKoVecs/derekovecs-server.c
+++ b/lib/IDS/DeReKoVecs/derekovecs-server.c
@@ -70,7 +70,6 @@
 } profile_t;
 
 float *M, *M2 = 0L, *syn1neg_window, *expTable;
-float *window_sums;
 char *vocab;
 char *garbage = NULL;
 COLLOCATORDB *cdb = NULL;
@@ -260,11 +259,18 @@
     expTable[i] = exp((i / (float)EXP_TABLE_SIZE * 2 - 1) * MAX_EXP);  // Precompute the exp() table
     expTable[i] = expTable[i] / (expTable[i] + 1);                     // Precompute f(x) = x / (x + 1)
   }
-  window_sums = malloc(sizeof(float) * (window + 1) * 2);
 
   return 0;
 }
 
+/* The collocator threads of one request accumulate their activation sums per
+   window position here. This must not be shared between requests, so every
+   request allocates its own array. Only valid once init_net() has determined
+   the window size, i.e. if M2 is set. */
+float *new_window_sums() {
+  return calloc((window + 1) * 2, sizeof(float));
+}
+
 long mergeVectors(char *file_name) {
   FILE *f;
   int binwords_fd, binvecs_fd;
@@ -782,6 +788,7 @@
 
 SV *getPosWiseW2VCollocators(char *word, long maxPerPos, long cutoff, float threshold, const char *format) {
   float *target_sums = NULL;
+  float *window_sums = NULL;
   long a, b, entries = 0;
   knn *syn_nbs[MAX_THREADS];
   knnpars pars[MAX_THREADS];
@@ -804,6 +811,7 @@
   }
 
   pt = (pthread_t *)malloc((num_threads + 1) * sizeof(pthread_t));
+  window_sums = new_window_sums();
   a = posix_memalign((void **)&target_sums, 128, cutoff * sizeof(float));
   memset(target_sums, 0, cutoff * sizeof(float));
 
@@ -853,6 +861,7 @@
 
   free(result);
   free(target_sums);
+  free(window_sums);
   free(pt);
   free(wl);
   for (a = 0; a < syn_threads; a++) free_knn(syn_nbs[a]);
@@ -871,6 +880,7 @@
 SV *get_neighbours(char *st1, int N, int sort_by, int search_backw, long cutoff, int dedupe, int no_similar_profiles) {
   HV *result = newHV();
   float *target_sums = NULL;
+  float *window_sums = NULL;
   long a, b, c, d, slice;
   knn *para_nbs[MAX_THREADS];
   knn *syn_nbs[MAX_THREADS];
@@ -919,6 +929,7 @@
     pthread_create(&pt[a], NULL, _get_neighbours, (void *)&pars[a]);
   }
   if (M2) {
+    window_sums = new_window_sums();
     for (a = 0; a < syn_threads; a++) {
       pars[a + para_threads].cutoff = cutoff;
       pars[a + para_threads].target_sums = target_sums;
@@ -1207,6 +1218,7 @@
 end:
   free(best);
   free(target_sums);
+  free(window_sums);
   free(pt);
   free(wl);
   for (a = 0; a < MAX_THREADS; a++) {