diff --git a/src/variable.cpp b/src/variable.cpp
index eb1deaae15..9a8236fdb4 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -1234,6 +1234,9 @@ double Variable::evaluate(char *str, Tree **tree, int ivar)
   int i = 0;
   int expect = ARG;
 
+  if (str == nullptr)
+    print_var_error(FLERR,"Invalid syntax in variable formula",ivar);
+
   while (1) {
     onechar = str[i];
 
@@ -4688,9 +4691,11 @@ void Variable::atom_vector(char *word, Tree **tree,
    max allowed # of args = MAXFUNCARG
 ------------------------------------------------------------------------- */
 
-int Variable::parse_args(char *str, char **args)
+int Variable::parse_args(const std::string &str, char **args)
 {
   int   narg = 0;
+  args[0] = nullptr;
+
   Tokenizer values(str,",");
 
   while (values.has_next() && narg < MAXFUNCARG) {
diff --git a/src/variable.h b/src/variable.h
index 5dc8748936..50d276bb14 100644
--- a/src/variable.h
+++ b/src/variable.h
@@ -122,7 +122,7 @@ class Variable : protected Pointers {
                       Tree **, Tree **, int &, double *, int &);
   int is_atom_vector(char *);
   void atom_vector(char *, Tree **, Tree **, int &);
-  int parse_args(char *, char **);
+  int parse_args(const std::string &, char **);
   void print_var_error(const std::string &, int, const std::string &,
                        int, int global=1);
   void print_tree(Tree *, int);
diff --git a/unittest/commands/test_variables.cpp b/unittest/commands/test_variables.cpp
index cc20ea2b96..90a0fe00e7 100644
--- a/unittest/commands/test_variables.cpp
+++ b/unittest/commands/test_variables.cpp
@@ -37,8 +37,8 @@ const bool have_openmpi = true;
 const bool have_openmpi = false;
 #endif
 
-using LAMMPS_NS::utils::split_words;
 using LAMMPS_NS::MathConst::MY_PI;
+using LAMMPS_NS::utils::split_words;
 
 namespace LAMMPS_NS {
 using ::testing::ExitedWithCode;
@@ -227,13 +227,13 @@ TEST_F(VariableTest, Expressions)
     ASSERT_FALSE(variable->equalstyle(ivar));
     ivar = variable->find("two");
     ASSERT_TRUE(variable->equalstyle(ivar));
-    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar),2.0);
+    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar), 2.0);
     ivar = variable->find("three");
-    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar),3.0);
+    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar), 3.0);
     ivar = variable->find("four");
-    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar),MY_PI);
+    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar), MY_PI);
     ivar = variable->find("five");
-    ASSERT_GE(variable->compute_equal(ivar),20210310);
+    ASSERT_GE(variable->compute_equal(ivar), 20210310);
 
     TEST_FAILURE(".*ERROR: Variable six: Invalid thermo keyword 'XXX' in variable formula.*",
                  command("print \"${six}\""););
@@ -249,14 +249,17 @@ TEST_F(VariableTest, Functions)
     command("variable one    index     1");
     command("variable two    equal     random(1,2,643532)");
     command("variable three  equal     atan2(v_one,1)");
+    command("variable four   equal     atan2()");
     if (!verbose) ::testing::internal::GetCapturedStdout();
-    ASSERT_EQ(variable->nvar, 3);
+    ASSERT_EQ(variable->nvar, 4);
 
     int ivar = variable->find("two");
-    ASSERT_GT(variable->compute_equal(ivar),0.99);
-    ASSERT_LT(variable->compute_equal(ivar),2.01);
+    ASSERT_GT(variable->compute_equal(ivar), 0.99);
+    ASSERT_LT(variable->compute_equal(ivar), 2.01);
     ivar = variable->find("three");
-    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar),0.25*MY_PI);
+    ASSERT_DOUBLE_EQ(variable->compute_equal(ivar), 0.25 * MY_PI);
+    TEST_FAILURE(".*ERROR: Variable four: Invalid syntax in variable formula.*",
+                 command("print \"${four}\""););
 }
 } // namespace LAMMPS_NS