diff --git a/libraries/AP_Common/Bitmask.cpp b/libraries/AP_Common/Bitmask.cpp
new file mode 100644
index 0000000000..9d0806cd40
--- /dev/null
+++ b/libraries/AP_Common/Bitmask.cpp
@@ -0,0 +1,27 @@
+#include "Bitmask.h"
+
+#include <AP_HAL/AP_HAL.h>
+
+Bitmask &Bitmask::operator=(const Bitmask&other)
+{
+    if (other.numwords != numwords || other.numbits != numbits) {
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        // we really should not be assigning from a bitmask of a
+        // different number of bits!
+        AP_HAL::panic("attempt to copy from bitmask of different size");
+#endif
+        // ... but try to cope if it happens in real life:
+        if (numwords != other.numwords) {
+            delete bits;
+            bits = new uint32_t[numwords];
+            numwords = other.numwords;
+        }
+        numbits = other.numbits;
+    }
+    if (other.numwords > 20) {
+        abort();
+    }
+    memcpy(bits, other.bits, 4*other.numwords);
+
+    return *this;
+}
diff --git a/libraries/AP_Common/Bitmask.h b/libraries/AP_Common/Bitmask.h
index 09e575a2ab..856257d468 100644
--- a/libraries/AP_Common/Bitmask.h
+++ b/libraries/AP_Common/Bitmask.h
@@ -33,7 +33,7 @@ public:
         delete[] bits;
     }
 
-    Bitmask &operator=(const Bitmask&other) = delete;
+    Bitmask &operator=(const Bitmask&other);
     Bitmask(const Bitmask &other) = delete;
 
     // set given bitnumber
@@ -119,7 +119,7 @@ public:
     }
 
 private:
-    const uint16_t numbits;
-    const uint16_t numwords;
+    uint16_t numbits;
+    uint16_t numwords;
     uint32_t *bits;
 };