diff --git a/src/core/buffer.cc b/src/core/buffer.cc
index 873a751ee5..829c43db1d 100644
--- a/src/core/buffer.cc
+++ b/src/core/buffer.cc
@@ -930,7 +930,7 @@ class Mmap_BufferImpl : public BufferImpl, MemoryMapWorker {
       for (size_t i = 0; i < n; ++i) {
         data[i] = Py_None;
       }
-      Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n);
+      Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + static_cast<Py_ssize_t>(n));
     }
     impl_->contains_pyobjects_ = true;
     return *this;
@@ -955,7 +955,7 @@ class Mmap_BufferImpl : public BufferImpl, MemoryMapWorker {
           if (n_new > n_old) {
             PyObject** data = static_cast<PyObject**>(xptr());
             for (size_t i = n_old; i < n_new; ++i) data[i] = Py_None;
-            Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n_new - n_old);
+            Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + static_cast<Py_ssize_t>(n_new - n_old));
           }
         } else {
           impl_->resize(newsize);
@@ -1019,7 +1019,7 @@ class Mmap_BufferImpl : public BufferImpl, MemoryMapWorker {
         size_t i = 0;
         for (; i < n_copy; ++i) Py_INCREF(newdata[i]);
         for (; i < n_new; ++i) newdata[i] = Py_None;
-        Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + n_new - n_copy);
+        Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + static_cast<Py_ssize_t>(n_new - n_copy));
       }
       impl_->release();  // noexcept
     }
diff --git a/src/core/column/const_na.cc b/src/core/column/const_na.cc
index 6ec7215ac2..d942ab76d9 100644
--- a/src/core/column/const_na.cc
+++ b/src/core/column/const_na.cc
@@ -85,7 +85,7 @@ static Column _fw_col(size_t nrows, SType stype) {
     });
 
   if (std::is_same<T, PyObject*>::value) {
-    Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + nrows);
+    Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + static_cast<Py_ssize_t>(nrows));
     buf.set_pyobjects(/* clear_data= */ false);
   }
   return Column(new ColClass(nrows, stype, std::move(buf)));
@@ -102,7 +102,7 @@ static Column _special_col(size_t nrows) {
     });
 
   if (std::is_same<T, PyObject*>::value) {
-    Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + nrows);
+    Py_SET_REFCNT(Py_None, Py_REFCNT(Py_None) + static_cast<Py_ssize_t>(nrows));
     buf.set_pyobjects(/* clear_data= */ false);
   }
   return Column(new ColClass(nrows, std::move(buf)));