diff --git a/src/freedv_interface.cpp b/src/freedv_interface.cpp
index a933b2ee..86152785 100644
--- a/src/freedv_interface.cpp
+++ b/src/freedv_interface.cpp
@@ -455,12 +455,16 @@ int FreeDVInterface::getTxSpeechSampleRate() const
 
 int FreeDVInterface::getTxNumSpeechSamples() const
 {
+    if (txMode_ == -1) return 1024;
+
     assert(currentTxMode_ != nullptr);
     return freedv_get_n_speech_samples(currentTxMode_);   
 }
 
 int FreeDVInterface::getTxNNomModemSamples() const
 {
+    if (txMode_ == -1) return 1024;
+
     assert(currentTxMode_ != nullptr);
     return freedv_get_n_nom_modem_samples(currentTxMode_);   
 }
@@ -603,6 +607,9 @@ IPipelineStep* FreeDVInterface::createTransmitPipeline(int inputSampleRate, int
     
     std::function<int(ParallelStep*)> modeFn = 
         [&](ParallelStep*) {
+            // Special handling for external vocoder.
+            if (txMode_ == -1) return 0;
+
             int index = 0;
             for (auto& dv : dvObjects_)
             {
diff --git a/src/pipeline/ExternVocoderStep.cpp b/src/pipeline/ExternVocoderStep.cpp
index ca0be14a..078baff5 100644
--- a/src/pipeline/ExternVocoderStep.cpp
+++ b/src/pipeline/ExternVocoderStep.cpp
@@ -128,7 +128,10 @@ std::shared_ptr<short> ExternVocoderStep::execute(std::shared_ptr<short> inputSa
     short output_buf[samplesToRead];
     short* inputPtr = inputSamples.get();
 
-    write(receiveStdinFd_, inputPtr, numInputSamples * sizeof(short));
+    if (numInputSamples > 0)
+    {
+        write(receiveStdinFd_, inputPtr, numInputSamples * sizeof(short));
+    }
     *numOutputSamples = read(receiveStdoutFd_, output_buf, samplesToRead * sizeof(short)); 
     if (*numOutputSamples == -1)
     {