diff --git a/ChangeLog b/ChangeLog
index bcd21011..a9196159 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2019-06-20  Jay Berkenbilt  <ejb@ql.org>
 
+	* Change out_bufsize argument to Pl_Flate's constructor for int to
+	unsigned int for compatibility with underlying zlib
+	implementation.
+
 	* Add methods to QPDFObjectHandle to return the value of Integer
 	objects as int and unsigned int with range checking and fallback
 	behavior to avoid silent underflow/overflow conditions.
diff --git a/include/qpdf/Pl_Flate.hh b/include/qpdf/Pl_Flate.hh
index fdd66996..923b33a8 100644
--- a/include/qpdf/Pl_Flate.hh
+++ b/include/qpdf/Pl_Flate.hh
@@ -27,13 +27,13 @@
 class Pl_Flate: public Pipeline
 {
   public:
-    static int const def_bufsize = 65536;
+    static unsigned int const def_bufsize = 65536;
 
     enum action_e { a_inflate, a_deflate };
 
     QPDF_DLL
     Pl_Flate(char const* identifier, Pipeline* next,
-	     action_e action, int out_bufsize = def_bufsize);
+	     action_e action, unsigned int out_bufsize = def_bufsize);
     QPDF_DLL
     virtual ~Pl_Flate();
 
@@ -43,11 +43,11 @@ class Pl_Flate: public Pipeline
     virtual void finish();
 
   private:
-    void handleData(unsigned char* data, int len, int flush);
+    void handleData(unsigned char* data, size_t len, int flush);
     void checkError(char const* prefix, int error_code);
 
     unsigned char* outbuf;
-    int out_bufsize;
+    size_t out_bufsize;
     action_e action;
     bool initialized;
     void* zdata;
diff --git a/libqpdf/Pl_Flate.cc b/libqpdf/Pl_Flate.cc
index 4cd48046..67e02d2d 100644
--- a/libqpdf/Pl_Flate.cc
+++ b/libqpdf/Pl_Flate.cc
@@ -1,13 +1,15 @@
 #include <qpdf/Pl_Flate.hh>
 #include <zlib.h>
 #include <string.h>
+#include <limits.h>
 
 #include <qpdf/QUtil.hh>
+#include <qpdf/QIntC.hh>
 
 Pl_Flate::Pl_Flate(char const* identifier, Pipeline* next,
-		   action_e action, int out_bufsize) :
+		   action_e action, unsigned int out_bufsize_int) :
     Pipeline(identifier, next),
-    out_bufsize(out_bufsize),
+    out_bufsize(QIntC::to_size(out_bufsize_int)),
     action(action),
     initialized(false)
 {
@@ -19,6 +21,13 @@ Pl_Flate::Pl_Flate(char const* identifier, Pipeline* next,
     // Windows environment.
     this->zdata = new z_stream;
 
+    if (out_bufsize > UINT_MAX)
+    {
+        throw std::runtime_error(
+            "Pl_Flate: zlib doesn't support buffer"
+            " sizes larger than unsigned int");
+    }
+
     z_stream& zstream = *(static_cast<z_stream*>(this->zdata));
     zstream.zalloc = 0;
     zstream.zfree = 0;
@@ -26,7 +35,7 @@ Pl_Flate::Pl_Flate(char const* identifier, Pipeline* next,
     zstream.next_in = 0;
     zstream.avail_in = 0;
     zstream.next_out = this->outbuf;
-    zstream.avail_out = out_bufsize;
+    zstream.avail_out = QIntC::to_uint(out_bufsize);
 }
 
 Pl_Flate::~Pl_Flate()
@@ -77,11 +86,17 @@ Pl_Flate::write(unsigned char* data, size_t len)
 }
 
 void
-Pl_Flate::handleData(unsigned char* data, int len, int flush)
+Pl_Flate::handleData(unsigned char* data, size_t len, int flush)
 {
+    if (len > UINT_MAX)
+    {
+        throw std::runtime_error(
+            "Pl_Flate: zlib doesn't support data"
+            " blocks larger than int");
+    }
     z_stream& zstream = *(static_cast<z_stream*>(this->zdata));
     zstream.next_in = data;
-    zstream.avail_in = len;
+    zstream.avail_in = QIntC::to_uint(len);
 
     if (! this->initialized)
     {
@@ -156,12 +171,13 @@ Pl_Flate::handleData(unsigned char* data, int len, int flush)
 		    // needed, so we're done for now.
 		    done = true;
 		}
-		uLong ready = (this->out_bufsize - zstream.avail_out);
+		uLong ready =
+                    QIntC::to_ulong(this->out_bufsize - zstream.avail_out);
 		if (ready > 0)
 		{
 		    this->getNext()->write(this->outbuf, ready);
 		    zstream.next_out = this->outbuf;
-		    zstream.avail_out = this->out_bufsize;
+		    zstream.avail_out = QIntC::to_uint(this->out_bufsize);
 		}
 	    }
 	    break;