diff --git a/src/libstore/build/worker.cc b/src/libstore/build/worker.cc
index ab0ba67b5..dbe86f43f 100644
--- a/src/libstore/build/worker.cc
+++ b/src/libstore/build/worker.cc
@@ -184,13 +184,13 @@ void Worker::wakeUp(GoalPtr goal)
 }
 
 
-unsigned Worker::getNrLocalBuilds()
+size_t Worker::getNrLocalBuilds()
 {
     return nrLocalBuilds;
 }
 
 
-unsigned Worker::getNrSubstitutions()
+size_t Worker::getNrSubstitutions()
 {
     return nrSubstitutions;
 }
diff --git a/src/libstore/build/worker.hh b/src/libstore/build/worker.hh
index 33a7bf015..e083dbea6 100644
--- a/src/libstore/build/worker.hh
+++ b/src/libstore/build/worker.hh
@@ -92,12 +92,12 @@ private:
      * Number of build slots occupied.  This includes local builds but does not
      * include substitutions or remote builds via the build hook.
      */
-    unsigned int nrLocalBuilds;
+    size_t nrLocalBuilds;
 
     /**
      * Number of substitution slots occupied.
      */
-    unsigned int nrSubstitutions;
+    size_t nrSubstitutions;
 
     /**
      * Maps used to prevent multiple instantiations of a goal for the
@@ -235,12 +235,12 @@ public:
      * Return the number of local build processes currently running (but not
      * remote builds via the build hook).
      */
-    unsigned int getNrLocalBuilds();
+    size_t getNrLocalBuilds();
 
     /**
      * Return the number of substitution processes currently running.
      */
-    unsigned int getNrSubstitutions();
+    size_t getNrSubstitutions();
 
     /**
      * Registers a running child process.  `inBuildSlot` means that