diff --git a/src/calibre/ebooks/compression/palmdoc.c b/src/calibre/ebooks/compression/palmdoc.c
index 4d913dfd2b..6b07bb9cd5 100644
--- a/src/calibre/ebooks/compression/palmdoc.c
+++ b/src/calibre/ebooks/compression/palmdoc.c
@@ -17,6 +17,7 @@
 #define BUFFER 6000
 
 #define MIN(x, y) ( ((x) < (y)) ? (x) : (y) )
+#define MAX(x, y) ( ((x) > (y)) ? (x) : (y) )
 
 typedef unsigned short int Byte;
 typedef struct {
@@ -53,7 +54,7 @@ cpalmdoc_decompress(PyObject *self, PyObject *args) {
     // Map chars to bytes
     for (j = 0; j < input_len; j++) 
         input[j] = (_input[j] < 0) ? _input[j]+256 : _input[j];
-    output = (char *)PyMem_Malloc(sizeof(char)*BUFFER);
+    output = (char *)PyMem_Malloc(sizeof(char)*(MAX(BUFFER, 5*input_len)));
     if (output == NULL) return PyErr_NoMemory();
 
     while (i < input_len) {