diff --git a/src/core/expr/funary/fexpr_isna.cc b/src/core/expr/funary/fexpr_isna.cc
index 66e9c4839..a57c40002 100644
--- a/src/core/expr/funary/fexpr_isna.cc
+++ b/src/core/expr/funary/fexpr_isna.cc
@@ -1,5 +1,5 @@
 //------------------------------------------------------------------------------
-// Copyright 2023 H2O.ai
+// Copyright 2022-2023 H2O.ai
 //
 // Permission is hereby granted, free of charge, to any person obtaining a
 // copy of this software and associated documentation files (the "Software"),
@@ -19,42 +19,37 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 // IN THE SOFTWARE.
 //------------------------------------------------------------------------------
-#include "expr/funary/fexpr_isna.h"
 #include "column/const.h"
 #include "column/isna.h"
+#include "expr/fexpr_column.h"
 #include "documentation.h"
+#include "expr/fexpr_func_unary.h"
 #include "expr/eval_context.h"
-#include "expr/fexpr_column.h"
 #include "expr/workframe.h"
 #include "python/xargs.h"
+#include "stype.h"
 namespace dt {
 namespace expr {
 
-FExpr_ISNA::FExpr_ISNA(ptrExpr &&arg) 
-    : arg_(std::move(arg)) 
-    {}
 
-std::string FExpr_ISNA::repr() const {
-  std::string out = "isna";
-  out += '(';
-  out += arg_->repr();
-  out += ')';
-  return out;
-}
+class FExpr_ISNA : public FExpr_FuncUnary {
+  public:
+    using FExpr_FuncUnary::FExpr_FuncUnary;
 
-Workframe FExpr_ISNA::evaluate_n(EvalContext &ctx) const {
-  Workframe wf = arg_->evaluate_n(ctx);
 
-  for (size_t i = 0; i < wf.ncols(); ++i) {
-    Column coli = wf.retrieve_column(i);
-    bool is_void_column = coli.stype() == SType::VOID;
-    coli = is_void_column? Const_ColumnImpl::make_bool_column(coli.nrows(), true)
-                         : Column(new Isna_ColumnImpl(std::move(coli)));
-    wf.replace_column(i, std::move(coli));
-  }
+    std::string name() const override {
+      return "isna";
+    }
+
+
+    Column evaluate1(Column&& col) const override{
+      bool is_void_column = col.stype() == SType::VOID;
+      col = is_void_column? Const_ColumnImpl::make_bool_column(col.nrows(), true)
+                          : Column(new Isna_ColumnImpl(std::move(col)));
+      return std::move(col);
+    }
+};
 
-  return wf;
-}
 
 static py::oobj pyfn_isna(const py::XArgs &args) {
   auto isna = args[0].to_oobj();
@@ -68,5 +63,5 @@ DECLARE_PYFN(&pyfn_isna)
     ->n_positional_args(1)
     ->n_required_args(1);
 
-} // namespace expr
-} // namespace dt
+
+}}  // dt::expr
diff --git a/src/core/expr/funary/fexpr_isna.h b/src/core/expr/funary/fexpr_isna.h
deleted file mode 100644
index 295758edc..000000000
--- a/src/core/expr/funary/fexpr_isna.h
+++ /dev/null
@@ -1,41 +0,0 @@
-//------------------------------------------------------------------------------
-// Copyright 2023 H2O.ai
-//
-// Permission is hereby granted, free of charge, to any person obtaining a
-// copy of this software and associated documentation files (the "Software"),
-// to deal in the Software without restriction, including without limitation
-// the rights to use, copy, modify, merge, publish, distribute, sublicense,
-// and/or sell copies of the Software, and to permit persons to whom the
-// Software is furnished to do so, subject to the following conditions:
-//
-// The above copyright notice and this permission notice shall be included in
-// all copies or substantial portions of the Software.
-//
-// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
-// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
-// IN THE SOFTWARE.
-//------------------------------------------------------------------------------
-#ifndef dt_EXPR_FEXPR_ISNA_h
-#define dt_EXPR_FEXPR_ISNA_h
-#include "expr/fexpr_func.h"
-namespace dt {
-namespace expr {
-
-
-class FExpr_ISNA : public FExpr_Func {
-  private:
-    ptrExpr arg_;
-
-  public:
-    FExpr_ISNA(ptrExpr&& arg);
-    std::string repr() const override;
-    Workframe evaluate_n(EvalContext& ctx) const override;
-};
-
-
-}}  // dt::expr
-#endif