Index: flang/lib/Evaluate/intrinsics.cpp
===================================================================
--- flang/lib/Evaluate/intrinsics.cpp
+++ flang/lib/Evaluate/intrinsics.cpp
@@ -1462,6 +1462,36 @@
   return true;
 }
 
+static void CheckMaxMinA1A2Argument(const ActualArguments &arguments,
+                                    std::set<parser::CharBlock> &set,
+                                    parser::ContextualMessages &messages) {
+  parser::CharBlock kwA1{"a1", 2};
+  parser::CharBlock kwA2{"a2", 2};
+  bool missingA1 = set.find(kwA1) == set.end();
+  bool missingA2 = set.find(kwA2) == set.end();
+
+  if (arguments[0]->keyword()) {
+    // If the keyword is specified in the first argument, the following
+    // arguments must have the keywords.
+    if (missingA1 && missingA2) {
+      messages.Say("missing mandatory '%s=' and '%s=' arguments"_err_en_US,
+                   kwA1.ToString(), kwA2.ToString());
+    } else if (missingA1 && !missingA2) {
+      messages.Say("missing mandatory '%s=' argument"_err_en_US,
+                   kwA1.ToString());
+    } else if (!missingA1 && missingA2) {
+      messages.Say("missing mandatory '%s=' argument"_err_en_US,
+                   kwA2.ToString());
+    }
+  } else if (arguments[1]->keyword()) {
+    // No keyword is specified in the first argument.
+    if (missingA1 && missingA2) {
+      messages.Say("missing mandatory '%s=' argument"_err_en_US,
+                   kwA2.ToString());
+    }
+  }
+}
+
 static bool CheckAtomicKind(const ActualArgument &arg,
     const semantics::Scope *builtinsScope,
     parser::ContextualMessages &messages) {
@@ -1523,6 +1553,11 @@
     } else if (isMaxMin) {
       if (CheckMaxMinArgument(arg->keyword(), maxMinKeywords, name, messages)) {
         actualForDummy.push_back(&*arg);
+
+        // max(x)/min(x) is invalid
+        if (arguments.size() < 2) {
+          actualForDummy.push_back(nullptr);
+        }
       } else {
         return std::nullopt;
       }
@@ -1570,6 +1605,10 @@
     }
   }
 
+  if (isMaxMin) {
+    CheckMaxMinA1A2Argument(arguments, maxMinKeywords, messages);
+  }
+
   std::size_t dummies{actualForDummy.size()};
 
   // Check types and kinds of the actual arguments against the intrinsic's
@@ -1590,7 +1629,22 @@
     const ActualArgument *arg{actualForDummy[j]};
     if (!arg) {
       if (d.optionality == Optionality::required) {
-        messages.Say("missing mandatory '%s=' argument"_err_en_US, d.keyword);
+        std::string kw{d.keyword};
+        if (isMaxMin && maxMinKeywords.size() == 1) {
+          // max(a1=x) or max(a2=x)
+          if (maxMinKeywords.begin()->ToString().compare("a1") == 0) {
+            messages.Say("missing mandatory '%s=' argument"_err_en_US, "a2");
+          } else if (maxMinKeywords.begin()->ToString().compare("a2") == 0) {
+            messages.Say("missing mandatory '%s=' argument"_err_en_US, "a1");
+          } else {
+            messages.Say(
+                "missing mandatory '%s=' and '%s=' arguments"_err_en_US, "a1",
+                "a2");
+          }
+        } else {
+          messages.Say("missing mandatory '%s=' argument"_err_en_US,
+                       kw.c_str());
+        }
         return std::nullopt; // missing non-OPTIONAL argument
       } else {
         continue;
Index: flang/test/Semantics/call23.f90
===================================================================
--- flang/test/Semantics/call23.f90
+++ flang/test/Semantics/call23.f90
@@ -1,6 +1,6 @@
 ! RUN: %python %S/test_errors.py %s %flang_fc1
 ! Check errors on MAX/MIN with keywords, a weird case in Fortran
-real :: x = 0.0 ! prevent folding
+real :: x = 0.0, y = 0.0 , y1 = 0.0 ! prevent folding
 !ERROR: Argument keyword 'a1=' was repeated in call to 'max'
 print *, max(a1=x,a1=1)
 !ERROR: Keyword argument 'a1=' has already been specified positionally (#1) in this procedure reference
@@ -9,4 +9,22 @@
 print *, max(x,0,a99=0) ! ok
 !ERROR: Argument keyword 'a06=' is not known in call to 'max'
 print *, max(a1=x,a2=0,a06=0)
+!ERROR: missing mandatory 'a2=' argument
+print *, max(a3=y, a1=x)
+!ERROR: missing mandatory 'a1=' argument
+print *, max(a3=y, a2=x)
+!ERROR: missing mandatory 'a1=' and 'a2=' arguments
+print *, max(a3=y, a4=x)
+!ERROR: missing mandatory 'a2=' argument
+print *, max(y1, a3=y)
+!ERROR: missing mandatory 'a1=' and 'a2=' arguments
+print *, max(a9=x, a5=y, a4=y1)
+!ERROR: missing mandatory 'a2=' argument
+print *, max(x)
+!ERROR: missing mandatory 'a2=' argument
+print *, max(a1=x)
+!ERROR: missing mandatory 'a1=' argument
+print *, max(a2=y)
+!ERROR: missing mandatory 'a1=' and 'a2=' arguments
+print *, max(a3=x)
 end