diff --git a/plink.c b/plink.c
index b7a2fd4b..b2c07acc 100644
--- a/plink.c
+++ b/plink.c
@@ -8,6 +8,7 @@
 #include <windows.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <assert.h>
 #include <stdarg.h>
 
 #define PUTTY_DO_GLOBALS	       /* actually _define_ globals */
@@ -147,6 +148,8 @@ int from_backend(int is_stderr, char *data, int len)
     HANDLE h = (is_stderr ? errhandle : outhandle);
     int osize, esize;
 
+    assert(len > 0);
+
     if (is_stderr) {
 	bufchain_add(&stderr_data, data, len);
 	try_output(1);
diff --git a/psftp.c b/psftp.c
index ead86140..094e68b0 100644
--- a/psftp.c
+++ b/psftp.c
@@ -1512,6 +1512,8 @@ int from_backend(int is_stderr, char *data, int datalen)
     unsigned char *p = (unsigned char *) data;
     unsigned len = (unsigned) datalen;
 
+    assert(len > 0);
+
     /*
      * stderr data is just spouted to local stderr and otherwise
      * ignored.
diff --git a/rlogin.c b/rlogin.c
index ba7dd37b..a80d12e3 100644
--- a/rlogin.c
+++ b/rlogin.c
@@ -68,7 +68,8 @@ static int rlogin_receive(Plug plug, int urgent, char *data, int len)
 	    }
 	    firstbyte = 0;
 	}
-	c_write(data, len);
+	if (len > 0)
+            c_write(data, len);
     }
     return 1;
 }
diff --git a/scp.c b/scp.c
index 60ca11cb..34f6ffb5 100644
--- a/scp.c
+++ b/scp.c
@@ -268,6 +268,8 @@ int from_backend(int is_stderr, char *data, int datalen)
     unsigned char *p = (unsigned char *) data;
     unsigned len = (unsigned) datalen;
 
+    assert(len > 0);
+
     /*
      * stderr data is just spouted to local stderr and otherwise
      * ignored.
diff --git a/terminal.c b/terminal.c
index 1c817dac..d76d7349 100644
--- a/terminal.c
+++ b/terminal.c
@@ -3603,6 +3603,8 @@ int term_ldisc(int option)
  */
 int from_backend(int is_stderr, char *data, int len)
 {
+    assert(len > 0);
+
     bufchain_add(&inbuf, data, len);
 
     /*