diff --git a/src/bitmask.cpp b/src/bitmask.cpp
index 4895b23..2e67953 100644
--- a/src/bitmask.cpp
+++ b/src/bitmask.cpp
@@ -105,9 +105,24 @@ namespace impl {
 PVXS_API
 void to_wire(Buffer& buf, const BitMask& mask)
 {
-    size_t nbytes = (mask.size() + 7u)/8u; // round up #bits to bytes
-    size_t nwords = nbytes / 8u;
-    size_t extra = nbytes % 8u; // trailing single bytes
+    // ignore trailing zeros
+    size_t nwords=mask.wsize();
+    size_t extra = 0u;
+    while(nwords) {
+        auto last = mask.word(nwords-1u);
+        if(last&0xff00000000000000ull) break;
+        nwords--;
+        if(last==0) continue;
+        else if(last&0x00ff000000000000ull) extra=7u;
+        else if(last&0x0000ff0000000000ull) extra=6u;
+        else if(last&0x000000ff00000000ull) extra=5u;
+        else if(last&0x00000000ff000000ull) extra=4u;
+        else if(last&0x0000000000ff0000ull) extra=3u;
+        else if(last&0x000000000000ff00ull) extra=2u;
+        else if(last&0x00000000000000ffull) extra=1u;
+        break;
+    }
+    size_t nbytes = nwords*8u + extra;
 
     to_wire(buf, Size{nbytes});
     for(auto i : range(nwords)) {
diff --git a/src/pvxs/bitmask.h b/src/pvxs/bitmask.h
index 75f95f6..8f3fdea 100644
--- a/src/pvxs/bitmask.h
+++ b/src/pvxs/bitmask.h
@@ -120,7 +120,7 @@ public:
     //! number of bits
     inline size_t size() const { return _size; }
     //! size()==0
-    inline bool empty() const { return _words.empty(); }
+    inline bool empty() const { return _size==0u; }
 
     //! number of storage words
     inline size_t wsize() const { return _words.size(); }
diff --git a/test/testbitmask.cpp b/test/testbitmask.cpp
index 692da05..663c08a 100644
--- a/test/testbitmask.cpp
+++ b/test/testbitmask.cpp
@@ -121,13 +121,13 @@ void testExpr()
 }
 
 template<size_t N>
-void testSerCase(bool be, uint8_t(&actual)[N], const char *expect)
+void testSerCase(bool be, uint8_t(&input)[N], const char *expect)
 {
-    std::string sactual((char*)actual, N-1u);
-    testShow()<<__func__<<"("<<(be?"BE":"LE")<<", \""<<escape(sactual)<<"\", \""<<expect<<"\")";
+    std::string sinput((char*)input, N-1u);
+    testShow()<<__func__<<"("<<(be?"BE":"LE")<<", \""<<escape(sinput)<<"\", \""<<expect<<"\")";
 
     BitMask mask;
-    FixedBuf inbuf(be, actual);
+    FixedBuf inbuf(be, input);
     from_wire(inbuf, mask);
 
     testEq(std::string(SB()<<mask), expect);
@@ -136,8 +136,8 @@ void testSerCase(bool be, uint8_t(&actual)[N], const char *expect)
     VectorOutBuf outbuf(be, O);
     to_wire(outbuf, mask);
 
-    testEq(sactual,
-           std::string((char*)O.data(), O.size()-outbuf.size()));
+    std::string actual((char*)O.data(), O.size()-outbuf.size());
+    testEq(sinput, actual);
 }
 
 void testSer()