diff --git a/internal/planner/planner.go b/internal/planner/planner.go
index 47b02f0606..9e791f7e8e 100644
--- a/internal/planner/planner.go
+++ b/internal/planner/planner.go
@@ -1090,12 +1090,21 @@ func (p *Planner) planUnifyVar(a ast.Var, b *ast.Term, iter planiter) error {
 	}
 
 	return p.planTerm(b, func() error {
-		target := p.newLocal()
-		p.vars.Put(a, target)
-		p.appendStmt(&ir.AssignVarStmt{
-			Source: p.ltarget,
-			Target: target,
-		})
+		// `a` may have become known while planning b, like in `a = input.x[a]`
+		la, ok := p.vars.GetOp(a)
+		if ok {
+			p.appendStmt(&ir.EqualStmt{
+				A: la,
+				B: p.ltarget,
+			})
+		} else {
+			target := p.newLocal()
+			p.vars.Put(a, target)
+			p.appendStmt(&ir.AssignVarStmt{
+				Source: p.ltarget,
+				Target: target,
+			})
+		}
 		return iter()
 	})
 }
diff --git a/test/cases/testdata/planner-ir/test-array-ir-unify.yaml b/test/cases/testdata/planner-ir/test-array-ir-unify.yaml
index ed016ac218..5ede0b0693 100644
--- a/test/cases/testdata/planner-ir/test-array-ir-unify.yaml
+++ b/test/cases/testdata/planner-ir/test-array-ir-unify.yaml
@@ -18,4 +18,29 @@ cases:
         [x | x := 1] = [foo]
       }
   want_result:
-  - x: 1
\ No newline at end of file
+  - x: 1
+- note: ir/fixpoint key/value (negative)
+  query: 'data.test.p = x'
+  input:
+    foos: ["foo"]
+  modules:
+    - |
+      package test
+      p {
+        some foo
+        foo == input.foos[foo]
+      }
+  want_result: []
+- note: ir/fixpoint key/value
+  query: 'data.test.p = x'
+  input:
+    foos: [2, 1, 0]
+  modules:
+    - |
+      package test
+      p = foo {
+        some foo
+        foo == input.foos[foo]
+      }
+  want_result:
+  - x: 1