diff --git a/src/libcmd/command.cc b/src/libcmd/command.cc
index c5b4aaacf1f..0df9a6882cd 100644
--- a/src/libcmd/command.cc
+++ b/src/libcmd/command.cc
@@ -313,7 +313,12 @@ MixEnvironment::MixEnvironment()
         .description = "Unset the environment variable *name*.",
         .category = environmentVariablesCategory,
         .labels = {"name"},
-        .handler = {[&](std::string s) { unsetVars.insert(s); }},
+        .handler = {[&](std::string name) {
+            if (setVars.contains(name))
+                throw UsageError("Cannot unset environment variable '%s' that is set with '%s'", name, "--set-env-var");
+
+            unsetVars.insert(name);
+        }},
     });
 
     addFlag({
@@ -327,9 +332,14 @@ MixEnvironment::MixEnvironment()
         .category = environmentVariablesCategory,
         .labels = {"name", "value"},
         .handler = {[&](std::string name, std::string value) {
+            if (unsetVars.contains(name))
+                throw UsageError(
+                    "Cannot set environment variable '%s' that is unset with '%s'", name, "--unset-env-var");
+
             if (setVars.contains(name))
                 throw UsageError(
                     "Duplicate definition of environment variable '%s' with '%s' is ambiguous", name, "--set-env-var");
+
             setVars.insert_or_assign(name, value);
         }},
     });