diff --git a/hclsyntax/expression.go b/hclsyntax/expression.go
index e81553b3..c4a353c4 100644
--- a/hclsyntax/expression.go
+++ b/hclsyntax/expression.go
@@ -252,6 +252,10 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 			}
 		}
 
+		extraUnknown := &functionCallUnknown{
+			name: e.Name,
+		}
+
 		// For historical reasons, we represent namespaced function names
 		// as strings with :: separating the names. If this was an attempt
 		// to call a namespaced function then we'll try to distinguish
@@ -274,6 +278,9 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				}
 			}
 
+			extraUnknown.name = name
+			extraUnknown.namespace = namespace
+
 			if len(avail) == 0 {
 				// TODO: Maybe use nameSuggestion for the other available
 				// namespaces? But that'd require us to go scan the function
@@ -291,6 +298,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 						Context:     e.Range().Ptr(),
 						Expression:  e,
 						EvalContext: ctx,
+						Extra:       extraUnknown,
 					},
 				}
 			} else {
@@ -308,6 +316,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 						Context:     e.Range().Ptr(),
 						Expression:  e,
 						EvalContext: ctx,
+						Extra:       extraUnknown,
 					},
 				}
 			}
@@ -331,6 +340,7 @@ func (e *FunctionCallExpr) Value(ctx *hcl.EvalContext) (cty.Value, hcl.Diagnosti
 				Context:     e.Range().Ptr(),
 				Expression:  e,
 				EvalContext: ctx,
+				Extra:       extraUnknown,
 			},
 		}
 	}
@@ -678,6 +688,27 @@ func (e *functionCallDiagExtra) FunctionCallError() error {
 	return e.functionCallError
 }
 
+// FunctionCallUnknownDiagExtra is an interface implemented by a value in the Extra
+// field of some diagnostics to indicate when the error was caused by a call to
+// an unknown function.
+type FunctionCallUnknownDiagExtra interface {
+	CalledFunctionName() string
+	CalledFunctionNamespace() string
+}
+
+type functionCallUnknown struct {
+	name      string
+	namespace string
+}
+
+func (e *functionCallUnknown) CalledFunctionName() string {
+	return e.name
+}
+
+func (e *functionCallUnknown) CalledFunctionNamespace() string {
+	return e.namespace
+}
+
 type ConditionalExpr struct {
 	Condition   Expression
 	TrueResult  Expression
diff --git a/hclsyntax/expression_typeparams_test.go b/hclsyntax/expression_typeparams_test.go
index 4ee9a77e..c7fdbd22 100644
--- a/hclsyntax/expression_typeparams_test.go
+++ b/hclsyntax/expression_typeparams_test.go
@@ -24,6 +24,71 @@ func TestExpressionDiagnosticExtra(t *testing.T) {
 		ctx    *hcl.EvalContext
 		assert func(t *testing.T, diags hcl.Diagnostics)
 	}{
+		// Errors for unknown function calls
+		{
+			"boop()",
+			&hcl.EvalContext{
+				Functions: map[string]function.Function{
+					"zap": function.New(&function.Spec{
+						Type: function.StaticReturnType(cty.String),
+						Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+							return cty.DynamicVal, fmt.Errorf("the expected error")
+						},
+					}),
+				},
+			},
+			func(t *testing.T, diags hcl.Diagnostics) {
+				t.Helper()
+				for _, diag := range diags {
+					extra, ok := hcl.DiagnosticExtra[FunctionCallUnknownDiagExtra](diag)
+					if !ok {
+						continue
+					}
+
+					if got, want := extra.CalledFunctionName(), "boop"; got != want {
+						t.Errorf("wrong called function name %q; want %q", got, want)
+					}
+					ns := extra.CalledFunctionNamespace()
+					if ns != "" {
+						t.Fatal("expected no namespace, got", ns)
+					}
+					return
+				}
+				t.Fatalf("None of the returned diagnostics implement FunctionCallUnknownDiagExtra\n%s", diags.Error())
+			},
+		},
+		{
+			"ns::source::boop()",
+			&hcl.EvalContext{
+				Functions: map[string]function.Function{
+					"zap": function.New(&function.Spec{
+						Type: function.StaticReturnType(cty.String),
+						Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+							return cty.DynamicVal, fmt.Errorf("the expected error")
+						},
+					}),
+				},
+			},
+			func(t *testing.T, diags hcl.Diagnostics) {
+				t.Helper()
+				for _, diag := range diags {
+					extra, ok := hcl.DiagnosticExtra[FunctionCallUnknownDiagExtra](diag)
+					if !ok {
+						continue
+					}
+
+					if got, want := extra.CalledFunctionName(), "boop"; got != want {
+						t.Errorf("wrong called function name %q; want %q", got, want)
+					}
+					ns := extra.CalledFunctionNamespace()
+					if ns != "ns::source::" {
+						t.Fatal("expected namespace ns::source::, got", ns)
+					}
+					return
+				}
+				t.Fatalf("None of the returned diagnostics implement FunctionCallUnknownDiagExtra\n%s", diags.Error())
+			},
+		},
 		// Error messages describing inconsistent result types for conditional expressions.
 		{
 			"boop()",