Index: flang/lib/Semantics/assignment.cpp
===================================================================
--- flang/lib/Semantics/assignment.cpp
+++ flang/lib/Semantics/assignment.cpp
@@ -216,10 +216,13 @@
   return true;
 }
 
-// 10.2.3.1(2) The masks and LHS of assignments must all have the same shape
+// 10.2.3.1(2) The masks and LHS of assignments must be arrays of the same shape
 void AssignmentContext::CheckShape(parser::CharBlock at, const SomeExpr *expr) {
   if (auto shape{evaluate::GetShape(foldingContext(), expr)}) {
     std::size_t size{shape->size()};
+    if (size == 0) {
+      Say(at, "The mask or variable must not be scalar"_err_en_US);
+    }
     if (whereDepth_ == 0) {
       whereExtents_.resize(size);
     } else if (whereExtents_.size() != size) {
Index: flang/test/Semantics/assign01.f90
===================================================================
--- flang/test/Semantics/assign01.f90
+++ flang/test/Semantics/assign01.f90
@@ -52,3 +52,26 @@
     end where
   end where
 end
+
+subroutine s4
+  integer :: x1 = 0, x2(2) = 0
+  logical :: l1 = .false., l2(2) = (/.true., .false./), l3 = .false.
+  !ERROR: The mask or variable must not be scalar
+  where (l1)
+    !ERROR: The mask or variable must not be scalar
+    x1 = 1
+  end where
+  !ERROR: The mask or variable must not be scalar
+  where (l1)
+    !ERROR: The mask or variable must not be scalar
+    where (l3)
+      !ERROR: The mask or variable must not be scalar
+      x1 = 1
+    end where
+  end where
+  !ERROR: The mask or variable must not be scalar
+  where (l2(2))
+    !ERROR: The mask or variable must not be scalar
+    x2(2) = 1
+  end where
+end
Index: flang/test/Semantics/assign04.f90
===================================================================
--- flang/test/Semantics/assign04.f90
+++ flang/test/Semantics/assign04.f90
@@ -185,17 +185,25 @@
   where ([1==1]) x='*'
   where ([1==1]) n='*' ! fine
   forall (j=1:1)
+    !ERROR: The mask or variable must not be scalar
     where (j==1)
       !ERROR: Defined assignment in WHERE must be elemental, but 'ctor' is not
+      !ERROR: The mask or variable must not be scalar
       x(j)='?'
+      !ERROR: The mask or variable must not be scalar
       n(j)='?' ! fine
+    !ERROR: The mask or variable must not be scalar
     elsewhere (.false.)
       !ERROR: Defined assignment in WHERE must be elemental, but 'ctor' is not
+      !ERROR: The mask or variable must not be scalar
       x(j)='1'
+      !ERROR: The mask or variable must not be scalar
       n(j)='1' ! fine
     elsewhere
       !ERROR: Defined assignment in WHERE must be elemental, but 'ctor' is not
+      !ERROR: The mask or variable must not be scalar
       x(j)='9'
+      !ERROR: The mask or variable must not be scalar
       n(j)='9' ! fine
     end where
   end forall